Reformat/redisign the Docker image creation.

[plague-doctor]

Thanks for your work! This is exactly what I was searching for.

A few changes to your dockerisation of BounCA:

Changes to Dockerfile:

• Prepare the building for automation. (Still waiting for https://gitlab.com/gitlab-org/gitlab/-/issues/421976 to be able to get the latest package).
• Reformat layers to save some space on the image (FYI: every RUN creates a new layer. If in one later you pull a file and in another layer you delete it, this doesn't change the size of the final image. Do all such operations within the same layer). This reformat saves almost 100MB in image size.

Changes to bounca-config.sh:

• Wait for Postgres to become available instead of quitting bounca when it is not available.
• Create Django Superuser when DJANGO_SUPERUSER_PASSWORD is specified.
• Rename/unify variables.

[nvollmar]

Thank you for the contribution. I'll review and merge it once I'm back home.

[nvollmar]

lgtm - thanks for the improvements