build(docker): add MySQL compose override for dialect-swap evaluation

Adds ``docker-compose-mysql.yml``, a compose-override file that swaps
the default Postgres metadata DB for MySQL 8 with one extra ``-f``
flag:

  docker compose -f docker-compose.yml -f docker-compose-mysql.yml up

Useful for evaluating dialect-specific behaviour (e.g., the runtime
cost of DDL migrations on a deployment whose production metadata DB
is MySQL — the question raised by review feedback on this PR).

Mirrors the connection settings used by CI's ``test-mysql`` shard:
``mysql+mysqldb`` dialect, charset ``utf8mb4`` with binary_prefix.
Host port defaults to 13306 (configurable via ``DATABASE_PORT_MYSQL``)
to avoid colliding with a native MySQL install on 3306.

A separate volume (``db_home_mysql``) keeps MySQL data isolated from
the Postgres ``db_home`` volume, so switching between the two with
``-f`` flag toggles doesn't corrupt either side.

The Postgres-specific init scripts under
``docker/docker-entrypoint-initdb.d/`` are not mounted on the MySQL
service (they are postgres-only). Examples / cypress fixtures still
load via ``superset-init``'s post-startup steps, which run
``superset load-examples`` against whichever metadata DB is in use.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>