Images

Think of an image as a read-only template which is a base foundation to create a container from. It's same as a recipe containing the definition of everything your application needs to operate. It can be Linux with an application server (such as Tomcat or Wildfly, for example) and your Java application itself. Every image starts from a base image; for example, Ubuntu; a Linux image. Although you can begin with a simple image and build your application stack on top of it, you can also pick an already prepared image from the hundreds available on the Internet. There are a lot of images especially useful for Java developers: openjdk, tomcat, wildfly, and many others. We will use them later as a foundation for our own images. It's a lot easier to have, let's say, Wildfly installed and configured properly as a starting point for your own image. You can then just focus on your Java application. If you're a novice in building images, downloading a specialized base image is a great way to get a serious speed boost in comparison to developing one by yourself.

Images are created using a series of commands, called instructions. Instructions are placed in the Dockerfile. The Dockerfile is just a plain text file, containing an ordered collection of root filesystem changes (the same as running a command that starts an application server, adding a file or directory, creating environmental variables, and so on.) and the corresponding execution parameters for use within a container runtime later on. Docker will read the Dockerfile when you start the process of building an image and execute the instructions one by one. The result will be the final image. Each instruction creates a new layer in the image. That image layer then becomes the parent for the layer created by the next instruction. Docker images are highly portable across hosts and operating systems; an image can be run in a Docker container on any host that runs Docker. Docker is natively supported in Linux, but has to be run in a VM on Windows and macOS. It's important to know that Docker uses images to run your code, not the Dockerfile. The Dockerfile is used to create the image when you run the docker build command. Also, if you publish your image to the Docker Hub, you publish a resulting image with its layers, not a source Dockerfile itself.

We have said before that every instruction in a Dockerfile creates a new layer. Layers are the internal nature of an image; Docker images are composed from them. Let's explain now what they are and what their characteristics are.