Update db.env

[frerich]

This adjusts the default db.env such that MATOMO_ variables are defined
in terms of MYSQL_ variables, if possible.

This avoids the need to duplicate the database name, user name or password,
which in turn avoids that minor typos cause a lot of confusion.

[frerich]

For what it's worth, I recently noticed that defining variables in terms of other variables does work on macOS using

• Docker version 20.10.2, build 2291f61
• docker-compose version 1.27.4, build 40524192

However, I recently noticed that it does not work on a Linux machine running

• Docker version 20.10.3, build 48d30b5
• docker-compose version 1.21.0, build unknown

With this setup, variable expansion didn't work, i.e. the PR broke stuff. So maybe this should be closed -- alas, it's not clear to me which bit makes this work (or break). Is it a matter of which shell is used? Which docker-compose version? Something else?