add_header will not override the Content-Type header set by the server,
but will instead add another header below, which is obviously not ideal.

The proposed change will instead tell nginx to set the correct value for
this header straight away.

The nginx instance which is serving the .well-known endpoints can serve
the simple JSON replies directly from memory, instead of having them
as external files on disk.

And mention that an empty string can be used to configure
Conduit purely with env vars.

Mention the need for a config.

- Dockerfile now tracks the gitlab repository and the master branch.
- docker-compose now points to conduit.toml instead of Rocket.toml and
its env vars were also renamed from ROCKET_ to CONDUIT_.
Furthermore vectorim/riot-web was changed to vectorim/element-web

...github.com repo, to relieve some pressure from git.koesters.xyz.