Support cache mounts in build steps

[cpuguy83]

It would be nice if build steps could include cache mounts (e.g. dockerfile's RUN --mount=type=cache,...). Originally we had this in the spec but I took it out since the only implementation (mariner2) could not support it.

Now once #115 is merged this is possible. Not to mention other implementations like debbuild which can also take advantage of it.