Reduce size of vMX by "sparsing" the vFPC image

[plajjan]

Looks like the vFPC image is 2.2GB on recent JUNOS. It's mostly empty and if it's converted to a sparse .qcow2 it takes like 100MB.

Problem is when can we do this? As soon as we copy the vfpc image its committed to a layer and then the data is there. It need to be done before the docker build but I don't want to rely on qemu-img being available to us.

I see docker has a new multi-stage build that could probably help us by allowing us to the run the build itself inside of a docker image while still not implicitly including all layers in the final output image.