Do-It-Yourself Images

Incidentally, Docker also helps users prepare containers for export to Docker. A container is usually nothing more than the entire filesystem of a Linux installation; Docker provides the ability to convert easily any directory to a Docker image. If you prefer a more spartan approach, you can, for example, create a lightweight system on a Debian or Ubuntu system using debootstrap. You simply install the dependencies for any application, install the application itself, and then hand over the folder to Docker when you're done. This process gives you an image that can be distributed freely. Building an image Docker-style is simple on top of that, because the docker program does this with a single command:

debootstrap precise ./rootfs; tar -C ./rootfs -c . | docker importubuntu/mybase

And, presto, you have an image.

The Goal: PaaS

With its portfolio of functions, Docker integrates very well with the current cloud computing and everything-as-a-service-dominated IT scene. Ultimately, Docker's goal is clear: By using prefabricated containers that can be distributed at will on the network, admins can easily take platform-as-a-service applications to the people. The PaaS concept suits Docker because it also focuses on the application rather than the operating system on which that application runs.

Additionally, a Docker Index service (Figure 1) now acts as a kind of container marketplace. Users who have built a container for a particular purpose can upload it to the index to make it available to other users. Participation in the Docker index is free; you just need to register.

Figure 1: The Docker developers want to use the index service to collect containers and make them available to users.

The number of different images that can be found in Docker is impressive. In addition to basic images for virtually all popular distributions, it includes ready-to-run custom images, such as multiple Tomcat images that are used to start an entire Java Tomcat instance at the click of a mouse. Docker images are already available for MySQL, Apache, or Drupal as prebuilt platforms and even for some OpenStack components. New images are being added every day. These choices help lower the barrier to entry: Once you have installed Docker, it does not take long to deploy your program.

Good Thinking: Automatic Build

For the Docker developers, PaaS obviously does not end when a user starts a container, subsequently to install their application in it. The Docker developers also want to facilitate this last step for users. Thus, an automated build system is integrated directly into Docker; it is aimed primarily at program developers who would like to distribute their apps as Docker containers. Dockerfiles provide the ability to configure a container in detail.

Developers can also use Dockerfiles to automate the process of creating Docker containers. This would be useful, for example, if you wanted to offer a snapshot version of your own application in addition to a stable version. In this case, the snapshot version would check out the current GitHub version of the program every night and build a complete Docker image from it. The Docker documentation [1] has more details on Dockerfiles.