Packer: Support docker image metadata (volumes, ports, labels, etc) and layers

[kapilt]

Without its not clear why one would use packer for docker since it leaves so many core features on the floor.

[mitchellh]

Good call, tagged.

[nhuisman]

I'd love to see at least a way to preserve previous layers that were inherited from a base image.

[davidkarlsen]

+1

[arnisoph]

+1

[cornfeedhobo]

For anyone watching, I made an updated Dockerfile handler as a work around.

https://github.com/cornfeedhobo/packer-post-processor-docker-dockerfile

[rickard-von-essen]

Support for this was just added in #4202