One container should ideally have one responsibility [1]. Therefore I
implemented the netbox-worker to start in it's own container. This is
possible, because netbox and the worker communicate via redis anyway.

They still use the same image underneath, just the "command" they
execute while starting different.

Or in other words: I see no reason to introduce supervisord, when we
already have docker-compose which can take care of running multiple
processes.

Also, here's another benefit: Now it's possible to view the logs of the
webhook worker independently of the other netbox logs (and vice-versa).

Other changes in this commit:
* I don't see a reason to put a password for Redis in the docker-compose
  setup, so I removed it.
* Slightly changed the nginx config, so that the nginx startup command
  becomes simpler and any error should be visible in the docker log.
* Some housekeeping in the `Dockerfile`.
* Added some troubleshooting advice regarding webhooks to the README.

I'd like to thank Brady (@bdlamprecht [2]) here who did the harder
work of figuring out what's even required to have webhooks working. [3]

[1] 
https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#decouple-applications
[2] https://github.com/bdlamprecht
[3] https://github.com/ninech/netbox-docker/pull/90