Modernize `docker-compose.yml`

[jsma]

• Moved database connection parameters and other environment variables to an environment file
• Added a persistent volume for the database (should solve #207)
• Removed the version number from docker-compose.yml since this is ignored by docker and causes IDEs to attempt to validate
• Added health checks for containers to avoid warnings/errors (and removed "try again" notes from the readme)
• Updated docs to use exec instead of run since the containers should already be running
• Updated base images to latest stable versions (Python 3.12, PostgreSQL 16, and Redis 7)
• Added missing notes about update_index

This was backported from my work in https://github.com/wagtail/docker-wagtail-develop/pull/71