Docker squash support

[lifeofguenter]

Seeing that docker build --squash is a experimental feature included in vanilla docker, it would be great if packer could support that option as well, instead of having to currently rely on external tools.

[hypery2k]

You can us it like that:

source "docker" "autogenerated_1" {
  changes = ["VOLUME /data", "WORKDIR /data", "ENTRYPOINT /bin/bash"]
  commit  = true
  squash  = true
...
}

[M8torr]

You can us it like that:

source "docker" "autogenerated_1" {
  changes = ["VOLUME /data", "WORKDIR /data", "ENTRYPOINT /bin/bash"]
  commit  = true
  squash  = true
...
}

This doesn't seem to work in the latest docker plugin. Was this ever implemented?