Create your own Docker image (Tech Tip #57)

Docker simplifies software delivery by making it easy to build and share images that contain your application’s entire environment, i.e. operating system, JDK, database, WAR file, specific tuning required for your application, etc.

There are three main components of Docker:

  • Docker images are “build component” – a read-only template of application operating system.
  • Containers are “run component” – a runtime representation created from images.
  • Registry are “distribution component” – a place to store and distribute images.

Several JBoss projects are available as Docker images at www.jboss.org/docker. Tech Tip #39 explained how to get started with Docker on Mac. It also explained how to start the official WildFly Docker image.

Docker image is made up of multiple layers where each layer provides some functionality, and a higher layer can add functionality on top of it. For example, Docker mounts the root filesystem as read-only layer and then adds a read-write layer on top of it. All these layers are combined together using Union Mount to provide application operating environment.

The complete history of how the WildFly image was built can be seen as:

The exact command issued at each layer is listed in this output. If you scroll to the far right then you can see the total space consumed by each layer as well. For example, Fedora is used as the base image and consumes ~574 MB of the total image, Open JDK 7 is taking 217.5 MB and WildFly is 135 MB.

Docker images are built by reading the instructions from Dockerfile. This is a text file that contains all the commands, in order, needed to build a given image. It adheres to a specific format and use a specific set of instructions. The vocabulary of commands is rather limited but serves the purpose well. The image can be built by giving the command docker build. Docker Tutorial provides complete instructions on how to create your own custom image.

The official WildFly Docker image is built using Fedora 20 as the base operating system. The Dockerfile can be seen at github.com/jboss-dockerfiles/wildfly/blob/master/Dockerfile. It uses  jboss/base-jdk:7 as the base image, which uses jboss/base as the base image. Dockerfile of jboss/base shows Fedora 20 is used as the base image.

An alternative is to build this image using CentOS or Ubuntu as a base image. Dockerfiles for these images are available at github.com/arun-gupta/docker-images/.

Starting boot2docker shows the output as:

And then you can build the CentOS-based WildFly Docker image as shown below. Note this command is given from the “wildfly-centos” directory of github.com/arun-gupta/docker-images/. And so the Dockerfile is at github.com/arun-gupta/docker-images/blob/master/wildfly-centos/Dockerfile.

The list of Docker images can now be seen as:

The total image size is 619.6 MB. The official WildFly Docker image can be installed as shown:

And the complete list of Docker images can again be seen as:

The image size in this case is 948.7 MB. A detailed understanding of this image is created was explained earlier in this blog.

Ubuntu-based WildFly image can be built and installed as shown below. Note this command is given from the “wildfly-ubuntu” directory of github.com/arun-gupta/docker-images/. And so the Dockerfile is at github.com/arun-gupta/docker-images/blob/master/wildfly-ubuntu/Dockerfile.

The list of Docker images can once again be seen as:

Docker image can run with docker run command. Some other related commands are:

  • docker ps: Lists containers
  • docker stop <id>: Stops the container with the given <id>

Run CentOS image as shown below. Specifying -i option will make it interactive and -t option allocates a pseudo-TTY. And port 8080 from the container is made accessible on port 80 of the container.

In a different shell, get the container’s IP address as:

And then access WildFly at http://192.168.59.103.

Similarly, running the WildFly Ubuntu image shows:

You can login to the host VM as shown:

Different layers of the image are stored in /var/lib/docker directory as shown:

VM image on Mac OSX is stored in ~/VirtualBox VMs/boot2docker-vm directory. This directory can grow up rather quickly if the intermediate containers are not removed. boot2docker-vm.vmdk on my machine is ~5GB for these different images.

You can reset it by running the following commands (WARNING: This will destroy all images you’ve downloaded and built so far):

Containers, as you can imagine, have a memory foot print.

More Docker goodness is coming in subsequent blogs!

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4 thoughts on “Create your own Docker image (Tech Tip #57)

  1. This article is perfect
    I just created a Docker image for JBoss Forge based on your references.

    Thanks!

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