Support arbitrary docker builders

[zimbatm]

Not everybody wants to use docker build to create docker images.

[zimbatm]

This might also require to use a custom uploading tools such as the libraries used by skopeo

[mloughran]

My feeling is that it would be simpler if linuxbox focussed on the problem of running your infrastructure and left the orthogonal problem of building containers to other tools. There are plenty of good solutions in that space from my perspective, and it doesn't seem necessary for linuxbox to handle this.