template.env and docker-compose.yml default to MySQL and phpMyAdmin #656

Problem

A new user who reaches for docker-compose.yml instead of the one-line quick start gets:

• template.env with database=mysql
• a MySQL container and a phpMyAdmin container, both started by default
• volume mounts that only make sense on the author's machine (../:/var/www/html/, ${PWD}/test.db)

That is a heavy and confusing starting point next to SQLite, which needs no second container and is what the install wizard selects by default. MySQL was also not working at all until #652.

Suggested fix

• Make the default compose file SQLite-only with one persistent data volume (see the quick-start persistence issue).
• Move MySQL and Postgres to commented-out or separate example files, and drop phpMyAdmin from the default.
• Make template.env match.

Found while scoping #651.