docker: Use mariadb-client, not mysql-client

docker: Use mariadb-client, not mysql-client

As noted in [1], use of mysql-client with mariadb-server can result in
the following errors:

  mysqldump: Couldn't execute 'SELECT COLUMN_NAME, JSON_EXTRACT(HISTOGRAM, '$."number-of-buckets-specified"')
  FROM information_schema.COLUMN_STATISTICS
  WHERE SCHEMA_NAME =3D 'patchwork' AND TABLE_NAME =3D 'auth_group';':
  Unknown table 'COLUMN_STATISTICS' in information_schema (1109)

Use the correct client for the backend.

[1] https://stackoverflow.com/a/67563944/613428

Signed-off-by: Stephen Finucane <stephen@that.guru>