error: missing JOOMLA_DB_HOST and MYSQL_PORT_3306_TCP environment variables
  Did you forget to --link some_mysql_container:mysql or set an external db
  with -e JOOMLA_DB_HOST=hostname:port?

-e JOOMLA_DB_HOST=... (defaults to the IP and port of the linked mysql container)
-e JOOMLA_DB_USER=... (defaults to "root")
-e JOOMLA_DB_PASSWORD=... (defaults to the value of the MYSQL_ROOT_PASSWORD environment variable from the linked mysql container)
-e JOOMLA_DB_NAME=... (defaults to "joomla")

$ docker run --name some-joomla -e JOOMLA_DB_HOST=10.1.2.3:3306
-e JOOMLA_DB_USER=... -e JOOMLA_DB_PASSWORD=... -d joomla