Dockerfile.test: split dev stage, and optimize order
Dockerfile.test: split dev stage, and optimize order

This makes the following changes:

- The containerd/config.toml, and docker-entrypoint.sh only occasionally change,
  so copy them before copying the source code to allow them to be cached.
- The cri-in-userns stage does not need files from proto3, so do not copy them
- The dev environment does need the file from the proto3 stage, so copy them there.
- Change the order of stages. Our CI uses `podman build` which (I think) does not
  skips stages that are not used for the specified target (like BuildKit does).
  So I moved stages that are not used for the `cri-in-userns` after that stage.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

dmcgowan/containerd,containerd/containerd,dmcgowan/containerd,estesp/containerd,thaJeztah/containerd,mikebrow/containerd,containerd/containerd,thaJeztah/containerd,mikebrow/containerd,estesp/containerd