Added MYSQL_UID and MYSQL_GID optional arguments / env. variables

[optimistiCli]

Passing uid and gid to “docker build” allows for consistent mysql user / group across host and image.

Also some hosts might have different (compared to debian) ideas about what uid / gid ranges are reserved for system users.

Hope this helps :-)

[optimistiCli]

Travis CI keeps failing on network timeouts. I guess it's time for me to give up trying to get through this hurdle :-)

[yosifkit]

This is unnecessary. Since #161 the image can be run as whatever user id is desired via --user and user: to docker run and in a docker compose file, respectively.

The only assumption is that if the container is given a local folder (-v /some/host/path:/var/lib/mysql), then the folder would need to have appropriate permissions for the user specified in the docker run or compose file.