Docker hub tags

[gamerlv]

Since #32 was merged the tags on docker hub have not been updated. I've opened #41 to update the readme here, but that does not change anything for people using the hub directly.

Should the description there be updated to point users to github and/or remove the tags to make it painfully obvious.
Removing the tags probably will break some builds, as some tags are still pulled. At time of writing latest was last pulled only 3 hours ago.

[balupton]

Yeah, I'd like to test https://github.com/bevry/dorothy against Elementary, especially for arm, however https://hub.docker.com/r/elementary/docker last updates were 3 years ago