metadata: disallow changing the runtime of an existing container
metadata: disallow changing the runtime of an existing container

This could render tasks for a container unresolvalbe. If there is a use
case for changing the runtime of a container, we should think it through
carefully.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

miaoyq/containerd,miaoyq/containerd,kunalkushwaha/containerd,dmcgowan/containerd,vdemeester/containerd,mikebrow/containerd,crosbymichael/containerd,containerd/containerd,thaJeztah/containerd,kunalkushwaha/containerd,estesp/containerd,dmcgowan/containerd,containerd/containerd,crosbymichael/containerd,vdemeester/containerd,mikebrow/containerd,estesp/containerd,thaJeztah/containerd,AkihiroSuda/containerd