Switch docker to collectively maintained Dockerfile

[jayvdb]

https://github.com/rroemhild/docker-errbot is maintained for all backends.

An optional part of switching to using that Docker is making our config.py consistent with the one in that repo, such as

• [ ] #371
• [ ] #370
• [x] #369

There is also a bit of work required in optimising the size of the docker at https://github.com/rroemhild/docker-errbot .