Automatically build and push multiarch docker images on Release

[adoy]

This new workflow is the second and last step (after #696) to release monstache for multiple architectures.

It will automatically generate docker images for latest, rel6 and also push images with semantic versionning. For example release v6.7.12 would push

monstache:latest
monstache:rel6
monstache:6
monstache:6.7
monstache:6.7.12

I did not use the base version of the image as you did previously, but based it directly on alpine. I think it make things more complicated, and since all the build is now done automatically the extra seconds are not a problem anymore.

You can find an exemple of the output here : https://hub.docker.com/r/adoy/monstache/tags (I will delete this repo once merged).

[adoy]

PS: The same workflow can be applied after that to handle rel5 and not to push lastest in this case, but I will submit a new PR for the rel5 branch once this one is approved.

[adoy]

@rwynn Remember to add those 2 variables (DOCKERHUB_USERNAME, DOCKERHUB_TOKEN) before triggering the build :)