Version docker images and use git tags

[AaronO]

Right now you have a separate docker image per go version. Unfortunately the images themselves are not versioned and all use the latest tag.

It would be nice to have a project level semver that is applied to all images, so one can chose a specific version of an image in the past and if it works be confident that it will never break in the future.

I encountered this because my cross compilation system (using xgo) suddenly broke with your latest releases, due to some obscure assembly error when compiling libwinhttp. Since there's only a latest, I'm unable to revert back to an older version of xgo at a given tag.

I think this would make xgo more reliable and production friendly.

I know a lot of people don't tag their docker images with versions or use semver, which is a shame.

[AaronO]

Btw, I was able to setup a 0.5.x branch, which uses the pre-xenial base image, which fixed my windows issues.

See branch diff here: AaronO/xgo@master...AaronO:v0.5.x