Docker Containers - The Heart of Navigation 💙

Posted Jan 10, 2025 Updated Jan 18, 2025

By Vishal Anarase

11 min read

Welcome back to Docking the Ship: Navigating Docker for Developers! In this installment, we’ll explore the cornerstone of Docker: containers. Containers have revolutionized the way we build, ship, and run applications. In this post, we’ll dive deep into what containers are, how they work, and why they’re so efficient.

What Are Docker Containers?

At its core, a Docker container is a lightweight, standalone, and executable package that includes everything needed to run an application: code, runtime, libraries, and dependencies. Containers are built on top of Docker images and provide an isolated environment for running applications.

How Containers Differ from Virtual Machines

While both containers and virtual machines (VMs) provide isolation, they do so in fundamentally different ways:

Feature	Containers	Virtual Machines
Isolation Level	Process-level	Full operating system-level
Size	Lightweight (megabytes)	Heavy (gigabytes)
Startup Time	Fast (seconds)	Slower (minutes)
Efficiency	Shares the host OS kernel	Requires a full guest OS

Containers achieve their efficiency by sharing the host operating system’s kernel, unlike VMs that require a full OS for each instance.

The Lifecycle of a Docker Container

Understanding the container lifecycle is key to managing them effectively. Here’s a breakdown:

Create
A container is created from a Docker image using the docker create command. At this stage, the container exists but isn’t running.

  
 $ docker create vishalanarase/navigating-docker:v1.0.0
 Unable to find image 'vishalanarase/navigating-docker:v1.0.0' locally
 v1.0.0: Pulling from vishalanarase/navigating-docker
 cb8611c9fe51: Already exists
 bd9615f269cd: Already exists
 39f8f326b044: Already exists
 859344cb3f85: Already exists
 4f4fb700ef54: Already exists
 2a927826cb18: Already exists
 bd2ff5f12cf5: Already exists
 9bcbe2868212: Already exists
 d2a3aa0d6430: Already exists
 eac0338a0e1d: Already exists
 Digest: sha256:902c518d2dd4d89e565ca96c9d20cc09faeca663486c2bb6683e3befad37fc06
 Status: Downloaded newer image for vishalanarase/navigating-docker:v1.0.0
 8674b559fc163fd2b107055d49bfa988e03965348548e349ec24d56aacc38930

Verify container called demo is created

  
 $ docker ps -a
 CONTAINER ID   IMAGE                                    COMMAND                  CREATED         STATUS       PORTS                               NAMES
 8674b559fc16   vishalanarase/navigating-docker:v1.0.0   "./webserver"            8 seconds ago   Created                                          demo

Start
The container is started with the docker start command, executing the process defined in the image.
1 2 $ docker start demo demo

Run
Alternatively, you can create and start a container in one step using docker run.

  
 $ docker run --name test -d vishalanarase/navigating-docker:v1.0.0
 82576266d26c8e68b4815a6a3628473fa0a1c94f5b3047134e58d44fa97071ec

Verify container called test is created

  
 $ docker ps
 CONTAINER ID   IMAGE                                    COMMAND                  CREATED          STATUS          PORTS                               NAMES
 82576266d26c   vishalanarase/navigating-docker:v1.0.0   "./webserver"            36 seconds ago   Up 35 seconds   8081/tcp                            test
 8674b559fc16   vishalanarase/navigating-docker:v1.0.0   "./webserver"            3 minutes ago    Up 2 minutes    8081/tcp                            demo

Pause/Unpause
Temporarily halt a container’s processes with docker pause and resume with docker unpause.
1 2 $ docker pause test test
1 2 $ docker unpause test test

Stop
Gracefully stop a running container using docker stop.

 $ docker stop test
 test

 $ docker start test
 test

Kill
Forcefully terminate a container with docker kill.
1 2 $ docker kill test test
Remove
Delete a container using docker rm. This action is final, so ensure you no longer need the container.
1 2 $ docker rm test test

Managing Docker Containers

Here are some essential commands for container management:

List Running Containers:

$ docker ps

$ docker ps
CONTAINER ID   IMAGE                                    COMMAND                  CREATED         STATUS             PORTS                               NAMES
8674b559fc16   vishalanarase/navigating-docker:v1.0.0   "./webserver"            6 minutes ago   Up 5 minutes       8081/tcp                            demo

List All Containers (Including Stopped):

$ docker ps -a

  
$ docker ps -a
CONTAINER ID   IMAGE                                    COMMAND                  CREATED         STATUS         PORTS                               NAMES
8674b559fc16   vishalanarase/navigating-docker:v1.0.0   "./webserver"            6 minutes ago   Up 5 minutes   8081/tcp                            demo

Inspect a Container:
View detailed information about a container:

$ docker inspect <container_id>

$ docker inspect demo

Output

  
[
    {
        "Id": "8674b559fc163fd2b107055d49bfa988e03965348548e349ec24d56aacc38930",
        "Created": "2025-01-09T15:44:13.915347501Z",
        "Path": "./webserver",
        "Args": [],
        "State": {
            "Status": "running",
            "Running": true,
            "Paused": false,
            "Restarting": false,
            "OOMKilled": false,
            "Dead": false,
            "Pid": 2285,
            "ExitCode": 0,
            "Error": "",
            "StartedAt": "2025-01-09T15:45:17.811367795Z",
            "FinishedAt": "0001-01-01T00:00:00Z"
        },
        "Image": "sha256:4e87789a83fc40d5144f6d5454acdc19cfb72a7072dece5497c0542c11222b85",
        "ResolvConfPath": "/var/lib/docker/containers/8674b559fc163fd2b107055d49bfa988e03965348548e349ec24d56aacc38930/resolv.conf",
        "HostnamePath": "/var/lib/docker/containers/8674b559fc163fd2b107055d49bfa988e03965348548e349ec24d56aacc38930/hostname",
        "HostsPath": "/var/lib/docker/containers/8674b559fc163fd2b107055d49bfa988e03965348548e349ec24d56aacc38930/hosts",
        "LogPath": "/var/lib/docker/containers/8674b559fc163fd2b107055d49bfa988e03965348548e349ec24d56aacc38930/8674b559fc163fd2b107055d49bfa988e03965348548e349ec24d56aacc38930-json.log",
        "Name": "/demo",
        "RestartCount": 0,
        "Driver": "overlay2",
        "Platform": "linux",
        "MountLabel": "",
        "ProcessLabel": "",
        "AppArmorProfile": "",
        "ExecIDs": null,
        "HostConfig": {
            "Binds": null,
            "ContainerIDFile": "",
            "LogConfig": {
                "Type": "json-file",
                "Config": {}
            },
            "NetworkMode": "bridge",
            "PortBindings": {},
            "RestartPolicy": {
                "Name": "no",
                "MaximumRetryCount": 0
            },
            "AutoRemove": false,
            "VolumeDriver": "",
            "VolumesFrom": null,
            "ConsoleSize": [
                45,
                158
            ],
            "CapAdd": null,
            "CapDrop": null,
            "CgroupnsMode": "private",
            "Dns": [],
            "DnsOptions": [],
            "DnsSearch": [],
            "ExtraHosts": null,
            "GroupAdd": null,
            "IpcMode": "private",
            "Cgroup": "",
            "Links": null,
            "OomScoreAdj": 0,
            "PidMode": "",
            "Privileged": false,
            "PublishAllPorts": false,
            "ReadonlyRootfs": false,
            "SecurityOpt": null,
            "UTSMode": "",
            "UsernsMode": "",
            "ShmSize": 67108864,
            "Runtime": "runc",
            "Isolation": "",
            "CpuShares": 0,
            "Memory": 0,
            "NanoCpus": 0,
            "CgroupParent": "",
            "BlkioWeight": 0,
            "BlkioWeightDevice": [],
            "BlkioDeviceReadBps": [],
            "BlkioDeviceWriteBps": [],
            "BlkioDeviceReadIOps": [],
            "BlkioDeviceWriteIOps": [],
            "CpuPeriod": 0,
            "CpuQuota": 0,
            "CpuRealtimePeriod": 0,
            "CpuRealtimeRuntime": 0,
            "CpusetCpus": "",
            "CpusetMems": "",
            "Devices": [],
            "DeviceCgroupRules": null,
            "DeviceRequests": null,
            "MemoryReservation": 0,
            "MemorySwap": 0,
            "MemorySwappiness": null,
            "OomKillDisable": null,
            "PidsLimit": null,
            "Ulimits": [],
            "CpuCount": 0,
            "CpuPercent": 0,
            "IOMaximumIOps": 0,
            "IOMaximumBandwidth": 0,
            "MaskedPaths": [
                "/proc/asound",
                "/proc/acpi",
                "/proc/kcore",
                "/proc/keys",
                "/proc/latency_stats",
                "/proc/timer_list",
                "/proc/timer_stats",
                "/proc/sched_debug",
                "/proc/scsi",
                "/sys/firmware",
                "/sys/devices/virtual/powercap"
            ],
            "ReadonlyPaths": [
                "/proc/bus",
                "/proc/fs",
                "/proc/irq",
                "/proc/sys",
                "/proc/sysrq-trigger"
            ]
        },
        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/4a212649d84f2d344c1004df1ac00315e3e6a9034fa9e9bf0b89e51bc95f501f-init/diff:/var/lib/docker/overlay2/rm08hgeeg2n1ull9sed0wg5lu/diff:/var/lib/docker/overlay2/qrs7mot5dzkubio5rzs7z7tuv/diff:/var/lib/docker/overlay2/xozzhsefrjwllxluwquhmw2cb/diff:/var/lib/docker/overlay2/4bdxfte72y7eakcz0a4eyplqb/diff:/var/lib/docker/overlay2/u3szopca12lfiy6yz42x6apzv/diff:/var/lib/docker/overlay2/df2496f5b13e01178dcfed8252695846450576f79b7831b70f44aac1fe8671d6/diff:/var/lib/docker/overlay2/feb7d81e23ee2e778286f5f5cb0d481569272894b928ed4fb338105c2f0e4434/diff:/var/lib/docker/overlay2/72a21c9133ef6499d8896650a1f3b6b37b3d410a8245392ffcd0cdbe366232df/diff:/var/lib/docker/overlay2/0bdd117f93f07c849f18ab3fb7d41f0bdf8c384857f9406fade4f9979e8a50a3/diff:/var/lib/docker/overlay2/faf20fcf861221030dc28e6d4874643ce562ff532ef2955423edc03f808c3a08/diff",
                "MergedDir": "/var/lib/docker/overlay2/4a212649d84f2d344c1004df1ac00315e3e6a9034fa9e9bf0b89e51bc95f501f/merged",
                "UpperDir": "/var/lib/docker/overlay2/4a212649d84f2d344c1004df1ac00315e3e6a9034fa9e9bf0b89e51bc95f501f/diff",
                "WorkDir": "/var/lib/docker/overlay2/4a212649d84f2d344c1004df1ac00315e3e6a9034fa9e9bf0b89e51bc95f501f/work"
            },
            "Name": "overlay2"
        },
        "Mounts": [],
        "Config": {
            "Hostname": "8674b559fc16",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": true,
            "AttachStderr": true,
            "ExposedPorts": {
                "8081/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "GOLANG_VERSION=1.23.4",
                "GOTOOLCHAIN=local",
                "GOPATH=/go"
            ],
            "Cmd": [
                "./webserver"
            ],
            "Image": "vishalanarase/navigating-docker:v1.0.0",
            "Volumes": null,
            "WorkingDir": "/app",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": {}
        },
        "NetworkSettings": {
            "Bridge": "",
            "SandboxID": "d8026809d87b8497945c50f86c6c2deab829682b4505f44594085294f538a9c4",
            "SandboxKey": "/var/run/docker/netns/d8026809d87b",
            "Ports": {
                "8081/tcp": null
            },
            "HairpinMode": false,
            "LinkLocalIPv6Address": "",
            "LinkLocalIPv6PrefixLen": 0,
            "SecondaryIPAddresses": null,
            "SecondaryIPv6Addresses": null,
            "EndpointID": "b106aff81c7a40396cd964c37e844e0b7bd3c1cd8fd380116eeb193a4a4949f2",
            "Gateway": "172.17.0.1",
            "GlobalIPv6Address": "",
            "GlobalIPv6PrefixLen": 0,
            "IPAddress": "172.17.0.2",
            "IPPrefixLen": 16,
            "IPv6Gateway": "",
            "MacAddress": "02:42:ac:11:00:02",
            "Networks": {
                "bridge": {
                    "IPAMConfig": null,
                    "Links": null,
                    "Aliases": null,
                    "MacAddress": "02:42:ac:11:00:02",
                    "DriverOpts": null,
                    "NetworkID": "12a23966fbbc258ec83b3b7abc6123d1e2f56212a55847466118f25f5889e686",
                    "EndpointID": "b106aff81c7a40396cd964c37e844e0b7bd3c1cd8fd380116eeb193a4a4949f2",
                    "Gateway": "172.17.0.1",
                    "IPAddress": "172.17.0.2",
                    "IPPrefixLen": 16,
                    "IPv6Gateway": "",
                    "GlobalIPv6Address": "",
                    "GlobalIPv6PrefixLen": 0,
                    "DNSNames": null
                }
            }
        }
    }
]

View Logs:
Check logs for debugging:

docker logs <container_id/name>

$ docker logs demo
Starting server on :8081

Access a Running Container:
Enter a container’s shell for troubleshooting:

docker exec -it <container_id/name> /bin/bash

  
$ docker exec -it demo sh

Run some basic commands

/app # ifconfig

Output

  
eth0      Link encap:Ethernet  HWaddr 02:42:AC:11:00:02
          inet addr:172.17.0.2  Bcast:172.17.255.255  Mask:255.255.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:65535  Metric:1
          RX packets:9 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:1046 (1.0 KiB)  TX bytes:0 (0.0 B)
  
lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

/app # ps aux
PID   USER     TIME  COMMAND
    1 root      0:00 ./webserver
   29 root      0:00 sh
   36 root      0:00 ps aux

/app # hostname
8674b559fc16

/app # hostname -i
172.17.0.2

/app # mount

Output

  
overlay on / type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay2/l/HJQ4PREEC2VKRCRTWEE75HSGCO:/var/lib/docker/overlay2/l/HAOXCKJQQKOHWM3BGG5YFUJ4X2:/var/lib/docker/overlay2/l/MVHX6WQSHAH5PKDE5VZ2YCWTLF:/var/lib/docker/overlay2/l/NBWXKYGBX6SY5MBUK6P2F7RKSQ:/var/lib/docker/overlay2/l/FXGSLQPK4GC6J4PCKSQT2ND4YR:/var/lib/docker/overlay2/l/T6YVWOBNC3RAKZHSXD5U2CDIQ6:/var/lib/docker/overlay2/l/FYEZEQJHBEQDGV55QSWJAMNLA3:/var/lib/docker/overlay2/l/UONY7FGWXP6PQJJEYI5ZQ64XGF:/var/lib/docker/overlay2/l/3WEIWPF5D3JNHIEKXLMZ2UQUPM:/var/lib/docker/overlay2/l/WNZY3IY7NPCJYVH2PZDDDNAOEJ:/var/lib/docker/overlay2/l/AUWBKUCGQMNCWX5FFSMQBEXSPW,upperdir=/var/lib/docker/overlay2/4a212649d84f2d344c1004df1ac00315e3e6a9034fa9e9bf0b89e51bc95f501f/diff,workdir=/var/lib/docker/overlay2/4a212649d84f2d344c1004df1ac00315e3e6a9034fa9e9bf0b89e51bc95f501f/work)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev type tmpfs (rw,nosuid,size=65536k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666)
sysfs on /sys type sysfs (ro,nosuid,nodev,noexec,relatime)
cgroup on /sys/fs/cgroup type cgroup2 (ro,nosuid,nodev,noexec,relatime)
mqueue on /dev/mqueue type mqueue (rw,nosuid,nodev,noexec,relatime)
shm on /dev/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=65536k)
/dev/vda1 on /etc/resolv.conf type ext4 (rw,relatime,discard)
/dev/vda1 on /etc/hostname type ext4 (rw,relatime,discard)
/dev/vda1 on /etc/hosts type ext4 (rw,relatime,discard)
proc on /proc/bus type proc (ro,nosuid,nodev,noexec,relatime)
proc on /proc/fs type proc (ro,nosuid,nodev,noexec,relatime)
proc on /proc/irq type proc (ro,nosuid,nodev,noexec,relatime)
proc on /proc/sys type proc (ro,nosuid,nodev,noexec,relatime)
proc on /proc/sysrq-trigger type proc (ro,nosuid,nodev,noexec,relatime)
tmpfs on /proc/kcore type tmpfs (rw,nosuid,size=65536k,mode=755)
tmpfs on /proc/keys type tmpfs (rw,nosuid,size=65536k,mode=755)
tmpfs on /proc/timer_list type tmpfs (rw,nosuid,size=65536k,mode=755)
tmpfs on /proc/scsi type tmpfs (ro,relatime)
tmpfs on /sys/firmware type tmpfs (ro,relatime)

  
/app # env
HOSTNAME=8674b559fc16
SHLVL=1
HOME=/root
GOTOOLCHAIN=local
TERM=xterm
PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
GOPATH=/go
PWD=/app
GOLANG_VERSION=1.23.4

/app # ip addr

IP Addresses </summary> ```bash 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: tunl0@NONE: mtu 1480 qdisc noop state DOWN qlen 1000 link/ipip 0.0.0.0 brd 0.0.0.0 3: gre0@NONE: mtu 1476 qdisc noop state DOWN qlen 1000 link/gre 0.0.0.0 brd 0.0.0.0 4: gretap0@NONE: <BROADCAST,MULTICAST> mtu 1462 qdisc noop state DOWN qlen 1000 link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff 5: erspan0@NONE: <BROADCAST,MULTICAST> mtu 1450 qdisc noop state DOWN qlen 1000 link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff 6: ip_vti0@NONE: mtu 1480 qdisc noop state DOWN qlen 1000 link/ipip 0.0.0.0 brd 0.0.0.0 7: ip6_vti0@NONE: mtu 1428 qdisc noop state DOWN qlen 1000 link/tunnel6 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 brd 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 8: sit0@NONE: mtu 1480 qdisc noop state DOWN qlen 1000 link/sit 0.0.0.0 brd 0.0.0.0 9: ip6tnl0@NONE: mtu 1452 qdisc noop state DOWN qlen 1000 link/tunnel6 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 brd 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 10: ip6gre0@NONE: mtu 1448 qdisc noop state DOWN qlen 1000 link/[823] 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 brd 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 37: eth0@if38: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 65535 qdisc noqueue state UP link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0 valid_lft forever preferred_lft forever ``` </details> ```bash /app # cat /etc/resolv.conf # Generated by Docker Engine. # This file can be edited; Docker Engine will not make further changes once it # has been modified. nameserver 192.168.65.7 # Based on host file: '/etc/resolv.conf' (legacy) # Overrides: [] ``` ```bash /app # ls Dockerfile LICENSE README.md go.mod go.sum main.go webserver ``` ```bash Try running binary again i.e. ./webserver /app # ./webserver Starting server on :8081 Error starting server: listen tcp :8081: bind: address already in use ``` ---

Container Isolation: Security and Efficiency

Docker containers provide isolation through the following mechanisms:

Namespaces
Namespaces ensure each container has its own isolated view of system resources (e.g., process IDs, file systems, and network interfaces).
Control Groups (cgroups)
Cgroups limit the resources (CPU, memory, disk I/O) a container can use, ensuring fair resource allocation.
Union File System (UnionFS)
UnionFS enables efficient storage by layering file systems. Changes made in a container are written to a new layer without modifying the base image.

Why Docker Containers Are So Efficient

Resource Sharing
Containers share the host OS kernel, reducing overhead and improving startup times.
Portability
Containers encapsulate all dependencies, ensuring consistent behavior across environments.
Scalability
Containers are lightweight, making it easy to scale applications horizontally by spinning up multiple instances.
Rapid Development and Deployment
With containers, developers can focus on writing code without worrying about environmental inconsistencies.

Conclusion

Docker containers are the heartbeat of modern application development. They provide a lightweight, efficient, and portable way to package and run applications. By mastering container lifecycles, management, and isolation, you’ll be well-equipped to harness their full potential.

In the next post, we’ll explore Docker Volumes : Persisting Data to understand how to manage persistent data and connect containers. Stay tuned, and happy docking! 🚢

Have questions or feedback? Drop them in the comments or connect with me. Let’s navigate the Docker seas together! 🌊

Docking the Ship Navigating Docker for Developers

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This post is licensed under CC BY 4.0 by the author.