Add docker support (closes #147)

[rfuehrer]

...I've been naughty ;)

I set up the docker branch as PR to show that the support is trouble-free and not very complex. The maintenance of these four files should only require a small effort in future releases.

I think the added value is very high and supports future CI/CD pipelines for automatic testing. Adjustments can be made by the user in the `.env' file. Updates regarding new versions of the services can be made by the contributors in the docker-compose.yml. However, this should be limited to changing the version numbers.

For further details I also have a version of the README, which explains the containers and the variables in detail. But in the current state the extension works without any adjustment.

Despite your answer I suggest the files in the hope to convince you to put the Docker support back to the upstream. If it still seems too complex, do whatever you want with them ;)

(in reference to #147)