Add the ability to override the gunicorn path in start scripts

[rooftopcellist]

No-Issue

What is this PR doing:

Make it possible to override the gunicorn path in the entrypoint command invocation.

readonly GUNICORN="${GUNICORN:-'/venv/bin/gunicorn'}"

It looks like gunicorn is not called with the full path in the content:

• https://github.com/pulp/pulp-oci-images/blob/latest/images/s6_assets/init/pulpcore-content#L13

And the worker doesn't seem to have any reference of gunicorn:

• https://github.com/ansible/galaxy_ng/blob/master/docker/bin/start-worker
• https://github.com/pulp/pulp-oci-images/blob/latest/images/s6_assets/init/pulpcore-worker

So I think it's just start-api and start-api-reload we need to worry about

Reviewers must know:

PR Author & Reviewers: Keep or remove backport labels per Backporting Guidelines
Reviewers: Look for sound code, no code smells, docs & test coverage
Merger: When merging, include the Jira issue link in the squashed commit

[jctanner]

start-content and start-content-reload should have these changes too.

[rooftopcellist]

This PR is ready to merge afaik. Please merge it when possible, thanks!