Deprecate the env-var way of running under docker

[richvdh]

We've decided to stop supporting the configuration of synapse via environment variables.

The main reason for this is that you're bound to end up needing to do more things than it's possible to do with the environment variables, and the incomplete support leads to problems like #5430.

An alternative might have been to make it genuinely possible to configure everything via environment variables in some consistent fashion, but that sounds hard and more work than we're able to do.

[richvdh]

For clarity: we'll provide documentation on how to convert your environment-variable-configured docker image to a yaml-configured one.

[richvdh]

done by #5566.