Improve --hostname=myhost to docker custom image

[sebyx07]

Changes

• allow --hostname=my-server to be passed

Issues

#5550

[sebyx07]

this helps me with sidekiq(to give a better name to the workers) and sentry. and I think will help others too

[andrasbacsai]

Thank you for the PR! I refactored a bit and now it works as it should.

[jonathan-reisdorf]

Thank you! Would there also be an option to do something like this:

--hostname=$(hostname)

for a multi-server setup? Currently, the hostname in the container is then a literal $(hostname). I can circumvent the issue though to a certain extent by mounting /etc/hostname:/etc/hostname:ro, which will not fully work as iirc it also needs to be passed to the kernel? but I can then inside the container at least access the hostname with cat /etc/hostname, export it to an environment var etc., so all good if it doesn't work :) Thanks!