Expose buildkit's cache mount functionality?

[maowtm]

This allows adding a mounted directory that persists between build runs. For example, we can mount target to such a cache, and cargo would not have to re-compile all the dependencies on each image build. (Currently this is achieved in our own Modusfile with a slight hack).

[maowtm]

Proposal: ::cached(path) (path can be relative or absolute, resolved based on cwd). Dir will initially just be empty.