Make Docker image available from registry

[eliskasl]

Instead of providing a Dockerfile for building an image locally, it would be helpful to make an actual image available from a registry, so that one can use it out-of-box and doesn't have to fight with rubygems and other issues along the way.

[jberkus]

A fine idea; I hadn't done it mostly because that dockerfile is kinda janky and I keep thinking I'll clean it up.

[mscherer]

So, we have now a s2i build system, see the .travis.yml ( https://github.com/projectatomic/atomic-site/blob/master/.travis.yml ).

Should we remove the Dockerfile all together ?