deploy: enable b2share backend, celery workers and celery beat to run in different containers

[nharraud]

Currently the B2Share backend, Celery workers and Celery beat processes are running inside the same b2share container. We should enable them to run in different containers.

Why:

• Celery workers and B2Share backends need to scale independently.
• There should be only one Celery beat service as it does not support high availability yet. Thus it should run in only one container.

The suggested design: Still have only one Dockerfile but with scripts running either B2Share backend, a Celery worker or Celery beat service depending on an environment variable. That way we still release only one container type but it can be used in different ways.

[nharraud]

I put this as hard because of the testing. The deployment scripts are easy to change.

[nharraud]

@llehtine this is the PR we discussed IRL.