Improve Traefik secuity settings, add security documentation

[jameswilliams1]

Description

Update Traefik settings to increase TLS strength, hide server software and enforce the same security Django uses for Flower (when it is used). Also add new documentation on security and explain any caveats in compatibilty caused by security settings.

Checklist:

• I've made sure that tests/test_cookiecutter_generation.py is updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

Fixes: #2841 fixes: #1992

The security settings used by Traefik were previously as default, this PR forces Traefik to use better TLS ciphers plus various other small security tweaks. When using docker, some HTTP secutity headers set by django are now set by Traefik so that they are also used by Flower (if celery is used) and any other parts of the site not served by Django.

Traefik has also been configured to run as non-root and to mount the traefik.yml rather than copy in the Dockerfile so that live reloading can be used when making changes.

Testing

I am running the exact same non-root Dockerfile in prod currently, no issues so far (I did see on the original issue about some kind of redirect issues in the logs but I am not seeing this?). Also am running the same TLS/headers settings, works as expected.

[jameswilliams1]

@browniebroke I think given there is already a fair bit of diff, CSP makes sense as its own issue. That would also need a lot of manual testing to ensure it doesn't block needed site features.

[browniebroke]

I haven't looked into details yet, but from a quick glance, it looks like some very solid work here @jameswilliams1!

Since this is touching with security settings, I don't want to rush the review and give it a good look.

One question I have (which might be a stupid one) is to know whether it makes sense to have some security settings both in Django and Traefik? You mention the Django deploy checks will raise false positives for example...

I think given there is already a fair bit of diff, CSP makes sense as its own issue.

Agree

[jameswilliams1]

@browniebroke yeah thats something I was deliberating with. On one hand I guess it's good to have a fallback, say if someone messes with the traefik.yml and removes the security and then django still catches it. But on the other config in 2 places gets confusing (as traefik will completely overwrite the django settings regardless of changes to production.py). I guess an option is to add the django settings back with a NOTE saying these are a fallback and won't affect anything when traefik is in use?

[Andrew-Chen-Wang]

Looks fine to me. Just a single concern.

[jameswilliams1]

@Andrew-Chen-Wang completely forgot about this PR, have just rebased and ci has passed now, could you review again?

[Andrew-Chen-Wang]

This LGTM now! I haven't tested this code due to limited resources. @browniebroke opinion?

[foxwoods369]

Shouldn't production.yml have the ports remapped for traefik?
e.g. "0.0.0.0:80:8080"