2. Quickstart

2.1. Dockerfile

All docker containers are defined through a Dockerfile. A Dockerfile has a set of predefined instructions or directives that declares how the container should be built, its state, as well as its runtime behavior. The official Dockerfile reference is avaible. The most important instruction is FROM, which defines which base image container your container will be based. Below, we the base image is busybox. The LABEL instruction specifies metadata in a key-value pair way for the container.

1FROM busybox
2
3LABEL "author"="One-Off Coder"
4LABEL "version"="0.0.1"
5LABEL "description"="A do nothing container."

The file may be downloaded.

2.2. Docker CLI

To build this container, we will use the docker commandline interface CLI. The docker CLI has many subcommands that you should be familiar with. Here are a few docker CLI subcommands commonly used.

  • build builds a docker image

  • run runs a docker image

  • ps is like the linux ps (process status) command and shows the container statuses

  • exec executes a command against a running container

  • stop stops a running container

  • rm removes a container

  • pull retrieves a docker image

  • push publishes a docker image

  • images lists all the images downloaded

  • login log into the Docker Hub repository.

  • search searches for docker images.

2.3. Build

Let’s build the docker container and type in the following. Note the options.

  • --no-cache specifies to not use the cache (always build)

  • -t specifies the tag, which is donothing:local

  • . specifies the root directory to start building from (the current)

1docker build --no-cache -t donothing:local .

You should see an output like the following.

Sending build context to Docker daemon  2.048kB
Step 1/4 : FROM busybox
---> 020584afccce
Step 2/4 : LABEL "author"="One-Off Coder"
---> Running in af8fdeea4744
Removing intermediate container af8fdeea4744
---> 8424b50d2202
Step 3/4 : LABEL "version"="0.0.1"
---> Running in 81f29ec03010
Removing intermediate container 81f29ec03010
---> d57b4186fb56
Step 4/4 : LABEL "description"="A do nothing container."
---> Running in 4208567a8939
Removing intermediate container 4208567a8939
---> a8f8fc8d975c
Successfully built a8f8fc8d975c
Successfully tagged donothing:local

2.4. Run

To run this container, type in the following.

1docker run donothing:local

Since this container does nothing, after running the command above, the container simply exits. You should check that the container indeed did run and exited.

1docker ps -a

Your output should resemble the following.

CONTAINER ID        IMAGE               COMMAND                  CREATED              STATUS                          PORTS               NAMES
42dcc9fe23b5        donothing:local     "sh"                     About a minute ago   Exited (0) About a minute ago

If we wanted to start this container and go into its shell, we may type in the following. Note the flags we pass into the run subcommand.

  • -i runs the container in interactive mode

  • -t creates a tty terminal

  • --rm automatically removes the container when it exits

1docker run -i -t --rm donothing:local /bin/sh

If we wanted to use this container to do something like echo or ping we may type in the following.

1docker run --rm donothing:local echo "hello, world!"

1docker run --rm donothing:local ping 8.8.8.8

2.5. Tagging

After you define and build a docker container, you may want to tag it for publication to a docker registry. To tag the container, type in the following.

1docker tag donothing:local oneoffcoder/donothing:0.0.1
2docker tag donothing:local oneoffcoder/donothing:latest

Note that we simply tag the already tagged image donothing:local to two new tags.

  • oneoffcoder/donothing:0.0.1

  • oneoffcoder/donothing:latest

Note the namespace oneoffcoder followed by a forward slash /. This tagging convention is to avoid tag collisions. Also, note that we specify two tag versions 0.0.1 and latest. The 0.0.1 follows semantic versioning, or semver, and the latest is to signal the latest version as well as default image that will be used when referenced without specifying a version (e.g. when pulling the image from a docker repository).

You may check to see if your tagging was successful by issuing the following command.

1docker images

Your output should look like the following.

REPOSITORY              TAG                 IMAGE ID            CREATED             SIZE
donothing               local               a8f8fc8d975c        36 minutes ago      1.22MB
oneoffcoder/donothing   0.0.1               a8f8fc8d975c        36 minutes ago      1.22MB
oneoffcoder/donothing   latest              a8f8fc8d975c        36 minutes ago      1.22MB
busybox                 latest              020584afccce        3 weeks ago         1.22MB
alpine                  <none>              f80194ae2e0c        10 months ago       4MB

2.6. Publishing

Now we are ready to publish to a docker repository. We will use Docker Hub as the repository, although you may create private ones on Amazon Web Services (AWS) or Azure. The docker repository service for these cloud service providers (CSPs) are shown below.

CSP Docker Repository Service

CSP

Service

AWS

Elastic Container Registry ECR

Azure

Azure Container Registry ACR

Before you may publish to Docker Hub, you need to create an account. After you create an account, you need to log into Docker Hub using the docker CLI.

1docker login

Now you may publish by typing in the following.

1docker push oneoffcoder/donothing:0.0.1
2docker push oneoffcoder/donothing:latest

2.7. Docker Hub

Just as in software engineering where the principle of code reuse is emphasized, in creating docker containers, it is also beneficial to reuse containers and/or build upon existing ones. Docker Hub has a plethora of published containers by major commercial vendors and products. You may use the search subcommand to search for existing images. In the example below, we search for containers related to Java.

1docker search java

You should get an output like the following.

NAME                                     DESCRIPTION                                     STARS               OFFICIAL            AUTOMATED
node                                     Node.js is a JavaScript-based platform for s…   8130                [OK]
tomcat                                   Apache Tomcat is an open source implementati…   2564                [OK]
openjdk                                  OpenJDK is an open-source implementation of …   1998                [OK]
java                                     Java is a concurrent, class-based, and objec…   1976                [OK]
ghost                                    Ghost is a free and open source blogging pla…   1062                [OK]
jetty                                    Jetty provides a Web server and javax.servle…   319                 [OK]
groovy                                   Apache Groovy is a multi-faceted language fo…   80                  [OK]
lwieske/java-8                           Oracle Java 8 Container - Full + Slim - Base…   45                                      [OK]
nimmis/java-centos                       This is docker images of CentOS 7 with diffe…   42                                      [OK]
fabric8/java-jboss-openjdk8-jdk          Fabric8 Java Base Image (JBoss, OpenJDK 8)      28                                      [OK]
frekele/java                             docker run --rm --name java frekele/java        12                                      [OK]
blacklabelops/java                       Java Base Images.                               8                                       [OK]
bitnami/java                             Bitnami Java Docker Image                       5                                       [OK]
cloudbees/java-with-docker-client        Java image with Docker client installed, use…   4                                       [OK]
rightctrl/java                           Oracle Java                                     3                                       [OK]
zoran/java10-sjre                        Slim Docker image based on AlpineLinux with …   2                                       [OK]
cfje/java-resource                       Java Concourse Resource                         1
cfje/java-buildpack                      Java Buildpack CI Image                         1
cfje/java-test-applications              Java Test Applications CI Image                 1
buildo/java8-wkhtmltopdf                 Java 8 + wkhtmltopdf                            1                                       [OK]
dwolla/java                              Dwolla’s custom Java image                      1                                       [OK]
jelastic/javaengine                      An image of the Java Engine server maintaine…   0
cfje/java-buildpack-dependency-builder   Java Buildpack Dependencies Builder Image       0
cfje/java-buildpack-memory-calculator    Java Buildpack Memory Calculator CI Image       0
thingswise/java-docker                   Java + dcd                                      0                                       [OK]

In nearly every case, it is encouraged to reuse existing container images.