Clarify PostgreSQL credentials in the Docker installation documentation

Hi,

I think the Docker installation documentation could use a small clarification regarding PostgreSQL credentials.

If a user changes POSTGRES_USER and/or POSTGRES_PASSWORD in the db service, the documentation should explicitly state that the corresponding:

PAPERLESS_DBUSER
PAPERLESS_DBPASS

must also be added to the environment: section of the webserver service in docker-compose.yml.

For example:

PAPERLESS_DBUSER: myuser
PAPERLESS_DBPASS: "mypassword"

This is not obvious to a new user, and without it Paperless fails with:

FATAL: password authentication failed for user "paperless"

Could this relationship, and the exact location where these variables should be added, be mentioned in the installation documentation?

Thanks!