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adding a new chapter for Docker images
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‎chapters/docker-basics.adoc‎

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[[Docker_Basics]]
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## Docker Basics
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*PURPOSE*: This chapter introduces the basic terminology of Docker.

‎chapters/docker-image.adoc‎

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== Docker Image
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*PURPOSE*: This chapter explains how to create a Docker image.
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As explained in <<Docker_Basics>>, Docker image is the *build component* of Docker and a read-only template of application operating system.
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=== Dockerfile
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Docker build images by reading instructions from a _Dockerfile_. A _Dockerfile_ is a text document that contains all the commands a user could call on the command line to assemble an image. `docker build` command uses this file and executes all the commands in succession to create an image.
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`build` command is also passed a context that is used during image creation. This context can be a path on your local filesystem or a URL to a Git repository.
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_Dockerfile_ is usually called _Dockerfile_. The complete list of commands that can be specified in this file are explained at https://docs.docker.com/reference/builder/. The common commands are listed below:
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.Common commands for Dockerfile
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[width="100%", options="header", cols="1,4,4"]
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|==================
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| Command | Purpose | Example
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| FROM | First non-comment instruction in _Dockerfile_ | `FROM ubuntu`
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| COPY | Copies mulitple source files from the context to the file system of the container at the specified path | `COPY .bash_profile /home`
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| ENV | Sets the environment variable | `ENV HOSTNAME=test`
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| RUN | Executes a command | `RUN apt-get update`
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| CMD | Defaults for an executing container | `CMD ["/bin/echo", "hello world"]`
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| EXPOSE | Informs the network ports that the container will listen on | `EXPOSE 8093`
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|==================
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=== Create your first Docker image
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_Hello World_ Dockerfile is shown:
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.Hello World Dockerfile
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[source, text]
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----
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FROM ubuntu
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CMD ["/bin/echo", "hello world"]
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----
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This image uses `ubuntu` as the base image. `CMD` command defines the command that needs to run. It provides a different entry point of `/bin/echo` and gives the argument "`hello world`".
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Build this image as:
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[source, text]
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----
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> docker build -t helloworld .
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Sending build context to Docker daemon 2.048 kB
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Step 0 : FROM ubuntu
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Pulling repository docker.io/library/ubuntu
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a5a467fddcb8: Download complete
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3fd0c2ae8ed2: Download complete
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9e19ac89d27c: Download complete
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ac65c371c3a5: Download complete
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Status: Downloaded newer image for ubuntu:latest
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---> a5a467fddcb8
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Step 1 : CMD /bin/echo hello world
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---> Running in 132bb0bf823f
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---> e81a394f71e3
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Removing intermediate container 132bb0bf823f
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Successfully built e81a394f71e3
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----
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List the images available:
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[source, text]
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----
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> docker images
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REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
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helloworld latest 9c0e7b56cbee 13 minutes ago 187.9 MB
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----
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Run the container:
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docker run -it helloworld
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to see the output:
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hello world
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Change the base image from `ubuntu` to `busybox`, build the image again:
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docker build -t helloworld2 .
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[source, text]
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----
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> docker images
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REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
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helloworld latest e81a394f71e3 26 minutes ago 187.9 MB
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helloworld2 latest c458787fadcf 3 seconds ago 1.113 MB
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ubuntu latest a5a467fddcb8 2 days ago 187.9 MB
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busybox latest 3d5bcd78e074 4 days ago 1.113 MB
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----
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==== Run WildFly
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Create a new text file _Dockerfile_ in an empty directory:
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[source, text]
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----
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FROM jboss/wildfly
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----
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Build the image:
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docker build -t mywildfly .
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Run the container:
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docker run -it mywildfly
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==== Deploy Java EE 7 Application
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Create a new text file _Dockerfile_ in an empty directory:
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[source, text]
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----
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FROM jboss/wildfly
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CMD ["/opt/jboss/wildfly/bin/standalone.sh", "-c", "standalone-full.xml", "-b", "0.0.0.0"]
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RUN curl -L https://github.com/javaee-samples/javaee7-hol/raw/master/solution/movieplex7-1.0-SNAPSHOT.war -o /opt/jboss/wildfly/standalone/deployments/movieplex7-1.0-SNAPSHOT.war
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----
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Three things happen in this image:
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. Uses "`jboss/wildfly`" as the base image
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. Starts WildFly application server in Full Platform mode
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. Copies the WAR file from from a URL to the deployment directory of WildFly
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Build the image:
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docker build -t movieplex .
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==== Difference between CMD and ENTRYPOINT
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*TL;DR* `CMD` will work for most of the cases.
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Default entry point for a container is `/bin/sh`, the default shell.
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Running a container as `docker run -it ubuntu` uses that command and starts the default shell. The output is shown as:
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[source, text]
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----
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> docker run -it ubuntu
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root@88976ddee107:/#
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----
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`ENTRYPOINT` allows to override the entry point to some other command, and even customize it. For example, a container can be started as:
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[source, text]
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----
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> docker run -it --entrypoint=/bin/cat ubuntu /etc/passwd
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root:x:0:0:root:/root:/bin/bash
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daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
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bin:x:2:2:bin:/bin:/usr/sbin/nologin
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sys:x:3:3:sys:/dev:/usr/sbin/nologin
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. . .
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----
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This command overrides the entry point to the container to `/bin/cat`. The argument(s) passed to the CLI are used by the entry point.
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==== Difference between ADD and COPY
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*TL;DR* `COPY` will work for most of the cases.
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`ADD` has all capabilities of `COPY` and has the following additional features:
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. Allows tar file auto-extraction in the image, for example, `ADD app.tar.gz /opt/var/myapp`.
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. Allows files to be downloaded from a remote URL. However, the downloaded files will become part of the image. This causes the image size to bloat. So its recommended to use `curl` or `wget` to download the archive explicitly, extract, and remove the archive.
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=== Import and export images
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Docker images can be saved using `save` command to a .tar file:
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docker save helloworld > helloworld.tar
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These tar files can then be imported using `load` command:
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docker load -i helloworld.tar
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‎readme.adoc‎

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include::chapters/docker-image.adoc[]
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include::chapters/docker-container.adoc[]
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include::chapters/docker-javaee7.adoc[]

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