docs(database): local development setup enabling the database on macOS

Related to #778 (closed)

This MR documents the process of setting up a local instance of Postgres using a container runtime on macOS and verifying that the setup works with an instance of the registry.

This might be easier to review in branch browser in https://gitlab.com/gitlab-org/container-registry/-/blob/778-document-database-setup-for-mac/docs-gitlab/database-local-setup.md