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Docker Basics

Table of Contents

1. Installing docker

  • There are multiple ways to install docker based on your platform, please read the docs https://docs.docker.com/engine/installation/

  • You can also get a VM on cloud provider like Amazon or Digital Ocean to make things easier for you.

1.1. Fedora

$ sudo dnf -y install docker
$ sudo systemctl start docker
$ sudo systemctl enable docker

For other Linux distributions or Windows , OS X, refer: https://www.docker.com/products/overview

2. Installing docker-compose

  • We will also look at how to dockerize an existing application and use docker-compose to bring multi-container application. To install docker-compose follow docs here: https://docs.docker.com/compose/install/

3. Check your docker installation

$ docker info
Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 2
Server Version: 1.10.3
Storage Driver: devicemapper
 Pool Name: docker-252:1-1574682-pool
 Pool Blocksize: 65.54 kB
 Base Device Size: 10.74 GB
 Backing Filesystem: xfs
 Data file: /dev/loop0
 Metadata file: /dev/loop1
 Data Space Used: 268.4 MB
 Data Space Total: 107.4 GB
 Data Space Available: 40.71 GB
 Metadata Space Used: 704.5 kB
 Metadata Space Total: 2.147 GB
 Metadata Space Available: 2.147 GB
 Udev Sync Supported: true
 Deferred Removal Enabled: false
 Deferred Deletion Enabled: false
 Deferred Deleted Device Count: 0
 Data loop file: /var/lib/docker/devicemapper/devicemapper/data
 WARNING: Usage of loopback devices is strongly discouraged for production use. Either use `--storage-opt dm.thinpooldev` or use `--storage-opt dm.no_warn_on_loop_devices=true` to suppress this warning.
 Metadata loop file: /var/lib/docker/devicemapper/devicemapper/metadata
 Library Version: 1.02.122 (2016-04-09)
Execution Driver: native-0.2
Logging Driver: journald
Plugins:
 Volume: local
 Network: host bridge null
Kernel Version: 4.7.3-200.fc24.x86_64
Operating System: Fedora 24 (Cloud Edition)
OSType: linux
Architecture: x86_64
Number of Docker Hooks: 2
CPUs: 4
Total Memory: 3.859 GiB
Name: fedora
ID: Y4OB:3DAQ:MMLV:M7RP:7UFB:KE2U:XAR3:KQIN:MDAY:TEDW:CVR6:WOGW
WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled
Registries: docker.io (secure)

4. docker Basic operations

4.1. Search in public docker index

$ docker search fedora
INDEX       NAME                                    DESCRIPTION                                     STARS     OFFICIAL   AUTOMATED
docker.io   docker.io/fedora                        Official Docker builds of Fedora                429       [OK]
docker.io   docker.io/fedora/apache                                                                 33                   [OK]
docker.io   docker.io/dockingbay/fedora-rust        Trusted build of Rust programming language...   3                    [OK]
docker.io   docker.io/gluster/gluster-fedora        Official GlusterFS image [ Fedora 21 + Glu...   3                    [OK]
docker.io   docker.io/startx/fedora                 Simple container used for all startx based...   2                    [OK]
docker.io   docker.io/dasrick/fedora-nginx          NGINX image - port 80, 443 - based on Fedo...   1                    [OK]
docker.io   docker.io/fedora/tools                  Docker image that has systems administrati...   1                    [OK]
docker.io   docker.io/mattsch/fedora-rpmfusion      Base container for Fedora with RPM Fusion ...   1                    [OK]
docker.io   docker.io/neroinc/fedora-apache         Plain and simple image with Apache httpd b...   1                    [OK]
docker.io   docker.io/pacur/fedora-22               Pacur Fedora 22                                 1                    [OK]

4.2. Pull an image

$ docker pull fedora:24
Trying to pull repository docker.io/library/fedora ...
24: Pulling from docker.io/library/fedora
Digest: sha256:64a02df6aac27d1200c2572fe4b9949f1970d05f74d367ce4af994ba5dc3669e
Status: Downloaded newer image for docker.io/fedora:24

$ docker pull alpine
Using default tag: latest
Trying to pull repository docker.io/library/alpine ...
latest: Pulling from docker.io/library/alpine
117f30b7ae3d: Pull complete
Digest: sha256:02eb5cfe4b721495135728ab4aea87418fd4edbfbf83612130a81191f0b2aae3
Status: Downloaded newer image for docker.io/alpine:latest

Try pulling a tagged image i.e. docker pull fedora:24

4.3. Listing images

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
docker.io/centos    latest              980e0e4c79ec        2 weeks ago         196.7 MB
docker.io/fedora    24                  11a5107645d4        5 weeks ago         204.4 MB
docker.io/fedora    latest              11a5107645d4        5 weeks ago         204.4 MB

4.4. Start a container from an image

$ docker run -it fedora bash
[root@1b65159da55c /]# cat /etc/fedora-release
Fedora release 24 (Twenty Four)

4.5. Listing the containers

Open another terrminal and run below command while running the container as mentioned in the previous state.

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
1b65159da55c        fedora              "bash"              35 seconds ago      Up 33 seconds                           silly_morse

To list all containers (both running and stopped)

$ docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS                      PORTS               NAMES
23e00a59a9b4        fedora              "ls"                6 seconds ago        Exited (0) 4 seconds ago                        adoring_knuth
1b65159da55c        fedora              "bash"              About a minute ago   Exited (0) 12 seconds ago                       silly_morse

4.6. Saving a container state

  • Start a container

  • Modify a file

]$ docker run -i -t fedora /bin/bash
[root@f369268c54cd /]# echo "docker 101" > /etc/motd

  • On a different terminal, save the container as an image

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
f369268c54cd        fedora              "/bin/bash"         37 seconds ago      Up 35 seconds                           amazing_turing

$ docker commit -a "Lalatendu Mohanty" -m "PyCon 2016" f369268c54cd pycon:motd
sha256:ce9d49c4ff59b756621dd18479a80fbb16eb25595949bc7ad0c1f22430c21397

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
pycon               motd                ce9d49c4ff59        16 seconds ago      204.4 MB
docker.io/centos    latest              980e0e4c79ec        2 weeks ago         196.7 MB
docker.io/fedora    24                  11a5107645d4        5 weeks ago         204.4 MB
docker.io/fedora    latest              11a5107645d4        5 weeks ago         204.4 MB

4.7. Looking in to docker metatdata

$ sudo cat /var/lib/docker/image/devicemapper/repositories.json  | python -mjson.tool
{
    "Repositories": {
        "docker.io/centos": {
            "docker.io/centos:latest": "sha256:980e0e4c79ec933406e467a296ce3b86685e6b42eed2f873745e6a91d718e37a",
            "docker.io/centos@sha256:2ae0d2c881c7123870114fb9cc7afabd1e31f9888dac8286884f6cf59373ed9b": "sha256:980e0e4c79ec933406e467a296ce3b86685e6b42eed2f873745e6a91d718e37a"
        },
        "docker.io/fedora": {
            "docker.io/fedora:24": "sha256:11a5107645d4ecb36e75d933576f5cdb52358bef385eac2c2d2a91af44ad4ad7",
            "docker.io/fedora:latest": "sha256:11a5107645d4ecb36e75d933576f5cdb52358bef385eac2c2d2a91af44ad4ad7",
            "docker.io/fedora@sha256:64a02df6aac27d1200c2572fe4b9949f1970d05f74d367ce4af994ba5dc3669e": "sha256:11a5107645d4ecb36e75d933576f5cdb52358bef385eac2c2d2a91af44ad4ad7"
        },
        "pycon": {
            "pycon:motd": "sha256:ce9d49c4ff59b756621dd18479a80fbb16eb25595949bc7ad0c1f22430c21397"
        }
    }
}

4.8. Attaching to a container

$ ID=$(sudo docker run -d fedora /bin/sh -c "while true; do echo PyCon 2016 ; sleep 1; done")
$ docker attach $ID
PyCon 2016
PyCon 2016
[SNIP]

4.9. Stop/Start/Restart a conatiner

$ docker stop $ID
$ docker start $ID
$ docker restart $ID

4.10. Looking at the logs of container

$ docker logs $ID

4.11. Killing a container

$ docker stop $ID
$ docker rm $ID

4.12. Delete all stopped containers

$ docker rm $(docker ps -aq)

4.13. Privileged access inside container

$ docker run -t -i fedora /bin/bash
[root@50559bf9ab0a /]# mount -t tmpfs none /mnt
mount: permission denied
[root@50559bf9ab0a /]# exit

To get privilaged access, please run below command

$ docker run --privileged -t -i fedora /bin/bash

4.14. Return low level information about container

$ docker inspect $ID
$ docker inspect --format='{{.NetworkSettings.IPAddress}}'  $ID

4.15. Check how much resources are used by the container

$ docker stats

4.16. Controlling resources used by a container

$ docker update  -m 512MB <ID>

P:S: You can get the container ID from "docker ps" command output

  • You can specify resources while running the container too. Check the docker run command usage.

$ docker run --help

4.17. Copy files/folders from a PATH on the container to a HOSTDIR on the host

$ docker cp $ID:/etc/motd /tmp/

4.18. Run a command inside a directory while starting a container

$ docker run -t -i -w /etc fedora ls

Note : if the path does not exist, it will get created

4.19. List the changed files and directories in a container’s filesystem

docker diff $ID

  • A Add

  • D Delete

  • C Change

4.20. History of the image

$ docker history

4.21. Networking

Bind a port to host interface

  • Bind TCP port 8080 of the container to TCP port 80 on 127.0.0.1 of the host machine.

$ docker run -d -i -t -p 127.0.0.1:8080:80 fedora bash

  • Bind TCP port 8080 of the container to a dynamically allocated TCP port on 127.0.0.1 of the host machine.

$ docker run -d -i -t -p 127.0.0.1::8080 fedora bash

  • Bind TCP port 8080 of the container to TCP port 80 on all available interfaces of the host machine.

docker run -d -i -t -p 80:8080 fedora bash

  • Bind TCP port 8080 of the container to a dynamically allocated TCP port on all available interfaces of the host machine.

docker run -d -i -t -p 8080 fedora bash

4.22. Exposing a port while starting a container

$ ID=$(docker run --expose=22 -d -i -t pycon:sshd /bin/bash)

4.23. Docker volumes

  • Any uncommitted data or changes in containers get lost as soon as containers are deleted

4.23.1. Data volumes

  • Data volumes persist even if the container itself is deleted.

$ docker run -i -t --name demo -v /data docker.io/fedora /bin/bash

Run below command in another terminal

$ docker inspect

4.23.2. Mount a host directory as a data volume

$ docker run  -i -t -v /var/logs:/logs_from_host:ro fedora bash
$ ls logs_from_host/

4.24. Remove docker image

docker rmi <imagename>

Remove all images

docker rmi $(docker images -q)

5. Dockerfiles

5.1. Building an image

$ mkdir /tmp/pycon; cd /tmp/pycon
$ echo "FROM fedora"  >> Dockerfile
$ echo "MAINTAINER Lalatendu" >> Dockerfile
$ docker build -t pycon/fedora .
$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
pycon/fedora        latest              8ab29ba8abf2        5 seconds ago       204.4 MB
pycon               motd                f6b39867b3e9        18 minutes ago      204.4 MB
docker.io/fedora    23                  5a813a9e051e        31 hours ago        214.4 MB

5.2. DockerFile Instructions

  • FROM <image> | <image>:<tag>

    Set the base image

  • MAINTAINER <name>

    Set the author

  • RUN <cmd> | ["executable", "param1", "param2"]

    Executes any commands in a new layer on top of the current image and commit the results

  • CMD ["executable","param1","param2"] | ["param1","param2"] | command param1 param2

    Provides defaults for an executing container

  • EXPOSE <port> [<port> …]

    Open up specified network ports at runtime

  • ENV <key> <value>

    This sets the environment variable <key> to the value <value>

  • ADD <src> <dest>

    Copy new files from source and add them to the container's filesystem at path

  • ENTYRPOINT ["executable", "param1", "param2"] | command param1 param2

    Helps to configure a container that you can run as an executable.

  • VOLUME ["/data"]

    Creates a mount point with the specified name and mark it as holding externally mounted volumes from native host or other containers.

  • USER

    Sets the username or UID to use when running the image.

  • WORKDIR

    Sets the working directory

  • ONBUILD [INSTRUCTION]

    Adds to the image a "trigger" instruction to be executed at a later time, when the image is used as the base for another build.