Use user name instead of userid ...

Nginx unit needs the user and group parameter as names.

Drops privileges to user 101 and group 0 ...

When the container is started as root the default was to drop privileges to "unit:unit". This caused some problems with temporary files. Now the privileges are drop to "101:0".
When the container is started as a normal user unit prints a warning that changing the user and group is not possible. This warning is safe to ignore.

Format shell scripts with shfmt

Is greater than or equal to is "-ge" and not "-gte".

Gunicorn is replaced with nginx-unit ...

We now serve Netbox with an nginx-unit instance instead of Gunicorn.
This allows us to get rid of the extra Nginx container because Unit is
also serving the static files. The static files are now collected at container
buildtime instead of every startup.