Because the development of Nginx Unit has been stopped, switch the server
to Granian which still allows to serve the Python application and the
static files from one server.
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.
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.
