Allow Docker container snapshots in Pipelines

Feedback from customer:

I’ve got a lot of npm modules under my command, all with varying npm dependencies – I’ve got a docker image with any npm global installs that are shared by all, but other than it’s not worth pinning down shared deps for the modules themselves. On a normal build server (and locally in dev), after the initial npm install, when dependencies change subsequent npm installs are very short. So, if I were setting it up myself from scratch in docker, I’d probably experiment with a docker commit at the end of a build, then keeping the images round for a week or so to speed up subsequent builds