[Core] Support image id when using docker as runtime environment

[cblmemo]

One user mentioned that pulling the docker container is slow, and one way to alleviate this is supporting image_id when docker as runtime env is used: we could use an image with a local docker cache. Currently we use image_id: docker:xxx to specify using docker as runtime env. Could consider:

resources:
  image_id:
    docker: <docker-image>
    <region>: <vm-image>

[Michaelvll]

Related to #2218