Optimize image size

[davidonlaptop]

Currently the adam and avocado images takes up to 1GB of disk space. It can be a pain to download an entire 1GB just to get a new feature of a few dozens lines of code...

There is good chances that 50% of this space can be spared by cleaning up after the build (e.g. maven artifacts, temporary files, etc).

E.g. we could use one docker container with maven to build an adam jar, and put the jar into another (smaller) container with java only.

[davidonlaptop]

Using a .dockerignore file could be of great help in reducing the final image size.

[frol]

Here are some points to keep an image as small as possible:

1. Use minimal base object (i.e. Alpine, which is 5MB!, or at least Debian instead of Ubuntu)
2. Use RUN command as less as possible - combine several actions into one by using && (here is a good example (though there should be also rm /var/cache/apt/archive/*): https://registry.hub.docker.com/u/gelog/java/dockerfile/ (why that image is way better then images in tis repo?))