docker worker update process

[kohlhase]

The software in a docker-based MathHub worker can be updated via lmh (or other means; see #5). But from time to time, the docker container itself will have to be updated, e.g. for a new version. We need to have a process that supports this, which is non-trivial since the MathHub worker contains configuration data (all the other data is on gl.mathhub.info and can be re-cloned and/or re-generated).

We need a process that exports all the config data that is not in the repositories in a form that the next worker can import it and re-create an analogous worker. If (see #6) the configuration data is in the repositories, the (AFAICT) only the list of libraries still needs to be ex/imported.

[kohlhase]

@jucovschi @cmaeder fyi