pluton/spawn: create containercache package
pluton/spawn: create containercache package

This allows creation of a bastion machine per test suite that other
machines can import containers directly from over the faster
inter-machine local network. This helps isolate and retry common
container downloads for bootkube testing that our often flaky while also
greatly reducing egress traffic of the test machines on a per suite
basis.

glevand/coreos--mantle,glevand/coreos--mantle,dm0-/mantle,dm0-/mantle