Use build args for versions

[dnephin]

Now that build args exist, it's a lot easier to do updates for this image.

I've made VERSION an arg, and the "check for existing" just look for the image instead of a git tag. That way we don't need git tags or to push to a git repo to do updates.

I think this will make it much easier to run as a jenkins job.

[aluzzardi]

Why RCs?

[dnephin]

We need to test against RCs, otherwise we end up with big surprises on release day.

Master is too volatile, and would result in a lot of failures that we can't do anything about.

[aluzzardi]

We do test against master in Swarm - we don't want to wait until RCs before finding out that the API is broken, which happens about once a month /cc @vieux

[dnephin]

Makes sense, we have slightly different requirements here. From Compose we only care about backwards compatibility and a few new features we plan on using, so we can wait for RC to find any breaking changes.

For Swarm you need to actually have API compatibility, so finding out about changes asap is important.

Are you still using this image for Swarm? I thought you were using dind-master exclusively ?

I have been uploading RC images to hub for a couple releases now.