Parameterize postgres db and user

[WaleedMortaja]

Ticket

https://github.com/opf/openproject-docker-compose

What are you trying to accomplish?

Continue the work made in #142: Creating more parameters in environment variables.

What approach did you choose and why?

The same approach, but I am not sure if you would prefer to allow this flexibility or not, so I made it as a separate PR.

IMPORTANT NOTE: There are documentations that will need to be changed if this gets merged, because the values for db or username are hard-coded there. An example that I faced, but not necessary the only one, is migration-to-postgresql17
I would suggest editing the commands to be like:

source .env
docker compose exec -it -u postgres db pg_dump -d ${POSTGRES_DB:-openproject} -U ${POSTGRES_USER:-postgres} -x -O > openproject.sql

Or assume the variables are always set?

source .env
docker compose exec -it -u postgres db pg_dump -d ${POSTGRES_DB} -U ${POSTGRES_USER} -x -O > openproject.sql

Side note: The quotation for DATABASE_URL makes source execute without misinterpretation (Otherwise, & in the URL creates a background job)

Merge checklist

• Added/updated tests
• Added/updated documentation in Lookbook (patterns, previews, etc)
• Tested major browsers (Chrome, Firefox, Edge, ...)