Support pushing images up to the registry

[vito]

We could really easily support pushing docker saved images, using these two methods:

• https://godoc.org/github.com/google/go-containerregistry/pkg/v1/tarball#ImageFromPath
• https://godoc.org/github.com/google/go-containerregistry/pkg/v1/tarball#Write

This would mesh really well with a Kaniko-based task for doing the actual building. Related: https://github.com/concourse/rfcs/issues/7 and concourse/docker-image-resource#190 (comment)

We'll probably need auth support for this first (#1).

[marco-m]

If I understand correctly there is preliminary support for pushing, for example in https://github.com/concourse/concourse/blob/master/ci/pipelines/concourse.yml:

  - put: unit-image
    params: {image: image/image.tar}

and the kaniko-based task is at https://github.com/concourse/builder, is this correct ?

Ah I saw the logs

WARN[0000] 'put' is experimental, untested, and subject to change! 

[vito]

Yep, right now it's still experimental as I haven't worked out how we want to write tests for it yet, and I needed it sooner for our own pipeline in order for the docker-image-less pipeline experiment to continue.

The builder task uses img, but otherwise correct.

[vito]

added a test for pushing - thank goodness for random.Image!

there is still much to be desired, namely being able to dynamically specify additional tags, but i've at least documented what (little) is there

the README also references the builder task