Consider support for basic environment variables in default config

[mwild1]

Currently the Docker image ships with the default Debian config, with the expectation that users of the image will override it with their own.

Duplicating all possible options as environment variables is not, and will not ever be, a goal. However some basic things such as default host name may be useful. Prosody 0.11 supports this via the ENV_* feature in the config file now.

[Leopere]

It would be pretty nice to see a set of envvars that can help people get a "works by default" docker-compose.yml there's nothing more annoying than software that's non-functional by default these days it's no longer the years before 2007.