✨ Use a default Redis password ...

Although it does not provide any additional security, it shows how to
configure Redis with a password and how to use Netbox using a password
protected redis server. Something that might be considered in a classic
production deployment. (But is mostly irrelevant in e.g. a Kubernetes /
OpenShift deployment as the isolation is usually on a network level.)

♻️ Make netbox-worker it's own container ...

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

✨ Disable webhooks by default* ...

*but enable it by default for anyone who checks out the netbox-docker project
via the netbox.env file.

Working implementation of webhooks using new 'redis' container

add allow all hosts by default

set MEDIA_ROOT to match docker-compose.yml ...

otherwise the default of `/etc/netbox/media` is used, which doesn't persist

Adding in support for NAPALM

❇️ Make the default configuration cluster ready ...

This changes the default configuration to be better prepared for
usage with container management platforms, such as Docker Swarm,
Kubernetes or OpenShift.

Closes #27.

✨ Creates a token for admin by default

🐞 Adds most common hostnames to ALLOWED_HOSTS ...

`nginx.netboxdocker.docker` and `netboxdocker.docker` are required
if our ['reception'][1] thing is used to access the container.

Fixes #21

[1]: https://github.com/ninech/reception

Restructured docker-compose.yml