make redis server connection configurable through $REDIS_URL

[lina-bh]

So that a Redis instance outside of the container may be used, read the REDIS_URL environment variable. If it is set and non-empty, assume it is a redis:// protocol URL and connect to that instead of forking a Redis instance inside the container. This allows the well understood behaviour of a dedicated Redis image, or a Redis running outside a container, to be used, including more flexible logging and control over configuration.

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[lina-bh]

I see that you have a new release coming up, so I'll wait for your lead on what to put in the changelog and where

[pSpitzner]

Hi @lina-bh,

thanks for the PR.

I think this is a good addition. I would make this part of 2.0.

Please base the PR on the release/2.0.0 branch, and just include a note about the new setting in the changelog (we will reformat it before the release, so anywhere within the 'added' section is fine)

Also, we should document it in docs/configuration.md, ideally with a small sample compose file to showcase the benefits.
Tbh I do not know how the external server will affect our init scripts that use rq worker commands, or the logging we apply to them, so the compose file will help me check it out.

best,
p