Docker Compose wiki example uses outdated DATABASE_URL environment variable

Hi! I noticed that the Docker Compose example in the wiki still uses the DATABASE_URL environment variable:

environment:
  - DATABASE_URL=postgres://...

However, recent versions of pgweb expect the connection string to be provided via PGWEB_DATABASE_URL instead.

So the example should be updated to:

environment:
  - PGWEB_DATABASE_URL=postgres://...

Otherwise, users following the current documentation may find that pgweb doesn’t automatically connect to the database.

Thanks for maintaining pgweb!