adding the ability to define a worker count for the http server

[tzezar]

Is your feature request related to a problem? Please describe.
currently there is 4 workers hardcoded

Describe the solution you'd like
env variable passed to docker

Describe alternatives you've considered

Additional context
4 workers might be bottleneck in some setups

[leonardehrenfried]

Sounds like a good contribution.

We could do something similar to what we do with the threads during import: https://github.com/mediagis/nominatim-docker/blob/master/5.1/init.sh#L10-L12

[tzezar]

#618