Add exim4 support

[b3yond]

exim4 is necessary to use the ticketfrei mailbot.

Unfortunately configuring it is non-trivial. How can we best configure it so users can just use some environment variables?

We should use more environment variables anyway... instead of letting users meddle with config files which are copied into the container at build time, we could use env vars for those values:

• domain (config.toml, traefik.toml)
• contact (config.toml, traefik.toml)

All the other config values should use the default option anyway when you use this docker-compose file.

[b3yond]

related: ticketfrei/ticketfrei#64

[b3yond]

Apparently you can't set these values through env variables in Traefik. So we can't eliminate the traefik.toml.