Allow Injection of ENV variables

[rafecolton]

into the Dockerfile. Allow them to be retrieved from the docker-builder process env

[jszwedko]

👏 🐣

[meatballhat]

👍 WANT

[jszwedko]

Another use case for this would be to inject the Travis CI build number. Use case would be as follows:

• Use https://github.com/travis-ci/docs-travis-ci-com/blob/3570751312e77df17ae3414e0e8cd30a624421a8/user/artifacts.md to have Travis upload the artifacts
• Refer to build and job number in Dockerfile to pull down the artifact from S3

[jszwedko]

I would vote for needed!

[rafecolton]

Removing "needed?" label, per feedback