Docker improvements

[romeobuilderotti]

• Added Docker Compose file and docs. Usually this way it's easier to start and manage than plain docker.
• Removed apt cache from the image, reducing its size
• Added --no-install-recommends that prevents adding extra recommended packages to the install list (and added ca-certificates package explicitly). This one might lead to errors if some required package is not explicitly installed but was in the recommended list of some other package. I've verified that current branch images successfully build and run, but this might require further testing. In any case it's better to specify any dependencies explicitly rather than relying on recommended packages.

Further improvements:

• Consider using alpine base image to considerably reduce image size.
• docker-compose.yml allows to add a TOR service using something similar to strm/tor. Easier setup will lead more users to run a TOR compatible queue in deployments by default.