Proposal: Support for building images with labels

[scic]

Currently docker-compose supports to add labels to containers. But we with pure docker we can add labels to images with docker build --label mylabel . (https://docs.docker.com/engine/reference/commandline/build/)
Since we recently also gained support for labelling networks and volumes: #3898 I suggest to implement this for building images as well.

It could look like this:

version: '2'
services:
  myservice:
    build:
      context: .
      labels:
        - mylabelOne= "labelTextOne"
        - mylabelTwo= "labelTextTwo"

[lpalgarvio]

Depending on this one.

[dm17]

Would be nice to know a bit more about how this works. I'm trying to add some labels that inform the image how to use networking during build time... However, they seem to get added as image layers after the container comes up & needs them for its build process.
Thanks!