Add docker-compose-prod-mysql.yml to freezing-compose to add additional operational flexibility

[obscurerichard]

It would be nice to give people the option of running mysql in Docker even in production. This could be done by adding one more file, docker-compose-prod-mysql.yml derived from docker-compose-dev.yml that would set it up a MySQL server for production use. The README.md documentation would need to be changed to acommodate that.