If you're installing as a system package, the system package should have set up
the systemd config, so it's more useful to give an example of running in a
virtualenv here.

Signed-off-by: Ruben Barkow <github@r.z11.de>

Signed-off-by: Jonas Schürmann <jonasschuermann@aol.de>

... and fix a typo

Link to docker/README.md

This addresses #3224

It's much easier to build the image via docker-compose instead of an
error-prone low-level docker call.

Signed-off-by: Benedikt Heine <bebe@bebehei.de>

Signed-off-by: Adrian Tschira <nota@notafile.com>