Improve documentation for docker images (contrib/clemux/docker)

[clemux]

The documentation for using clemux's docker images is for from being complete and should be improved.

[Shnatsel]

There's a naming discrepancy between the docs and actual files: "debile-postgresql.service" vs "postgresql-debile.service"

[EDIT by @clemux] : fixed in 549c57a

It would also help if the working directory was provided for all commands and not just the first few. Not really importnt, but would make the docs it a bit nicer to work with.