feat(registry): Allow FROM to refer to the off-cluster docker registry.

[robholland]

Authenticate to off-cluster registries before building so that the client is able to pull private images specified in the FROM.

Fixes #100

[deis-admin]

Thanks for the contribution! Please ensure your commits follow our style guide. This code will be tested once a Deis maintainer reviews it.

[deis-bot]

@kmala, @mboersma and @ultimateboy are potential reviewers of this pull request based on my analysis of git blame information. Thanks @robholland!

[bacongobbler]

IIRC there was a specific reason @kmala made it so we attach auth credentials when pushing rather than logging into the registry. @kmala, is there a reason for that logic or does this LGTY?

[bacongobbler]

If you wouldn't mind amending your commit such that it follows the commit style guide, that would be awesome! That way it'll get picked up by deisrel when we generate the release changelog. Thanks!

[robholland]

Yep, I'll reformat, sorry.

If it helps for testing the image is available at quay.io/clearbit/dockerbuilder:canary

[kmala]

Jenkins, add to Whitelist

[kmala]

I was assuming the login() will write the creds to the filesystem but thats not true and docker login does this explicitly https://github.com/docker/docker/blob/e8b9f38c7e2e249084c78c81f136c6d22432af6f/cli/command/registry/login.go#L77.

i had tested this PR by building/pushing an image and also verifying that i can't push the same image from the host on which the build happened.

[bacongobbler]

Attached the "needs rebase" label to note amending the commit first before merging.