Docker image for websocket server

[juliushaertl]

Relevant config options should already be configurable by environment variables.

• [x] Build a docker image for running the websocket server separately
• [x] Should have CI do automatically build it
• [x] Notify @szaimen about when it is ready

[juliushaertl]

Pushed a simple image now, we'll see if that needs some more finetuning during testing before automating the build.