Support for tags for each minor version of go

[sspeiche]

Currently only the tags 1 and latest are published to https://hub.docker.com/r/centos/go-toolset-7-centos7/tags/ though most Go applications need to be pinned to certain minor release numbers.

I'd suggest that this tag strategy should be used:

• latest - the version latest, follows 1 until 2 exists and then it would match that
• 1 - the latest 1.x version
• 1.11, 1.10, 1.9, etc - match the .x version of Go.