I'm facing the same problem.
The project uses Hibernate 7.1.2.Final, so you don't need to specify the dialect manually, but the error persisted.
The problem was that I forgot to put a colon in front of the two slashes.

Thus, make sure that the JDBC_DATABASE_URL is specified correctly.

If you are working with PostgreSQL, then the path will be as follows:

jdbc:postgresql://${HOSTNAME}:${DB_PORT}/${DATABASE}?password=${PASSWORD}&user=${USERNAME}

The example would look like this:

jdbc:postgresql://localhost:5432/taskdb?password=qwerty123&user=sa