Create experimental image for Swarm

[nishanttotla]

Based on @schmunk42's suggestion, I think it would be a good idea to have an :experimental tagged build (nightly) for swarm that has the latest features in the master branch.

This would be particularly nice since the Swarm releases are now decoupled from the Docker Engine releases.

[schmunk42]

Even better: Add some tags like 1.2.8-alpha1, 1.2.8-alpha2 from time to time. This would allow us this use an early version, but not taking the risk of getting unintended updates.

[nishanttotla]

@schmunk42 just FYI, I believe you can try new features with the dockerswarm/swarm:master image: https://hub.docker.com/r/dockerswarm/swarm/tags/

It might be sufficient to just add monthly or otherwise regular tags so that builds are triggered on that repo.

[schmunk42]

docker run dockerswarm/swarm --version
swarm version 1.2.8 (48d86b1)

@nishanttotla Looks not like the latest master