Docker-out-of-Docker support

[tomwatson1024]

We've got docker-in-docker support, but support for mounting and using the host's docker would be useful too - e.g. for building Docker images for use on the host.

[rlupton20]

I got to thinking about this, and one possible complication here is that the floki container and the host see different filesystems. For example running

docker run -v HOST_PATH:PATH ...

will mount HOST_PATH on the host, rather than in the floki container (although there are similar complications with dind, it's just likely you want to mount something in the source base which is at the same path in the dind container).

I'm not sure the tradeoff is worth it, but I can see the value in accessing the host.