Golang tip image

[gravis]

It would be nice to have a "tip" golang image, to test against nightly builds.
See: mattes/migrate#54
We need docker to make contributors life easier, but we're losing the ability to test against tip in travis.

[tianon]

This issue would be more appropriate over on https://github.com/docker-library/golang, but I'd note that the official images don't really have a mechanism to support automated builds, so the best we could do is a rough snapshot of Go tip periodicially, and that this is a request that's likely better served by some means other than the official images.