Support expanding env vars on the extra_build_args parameter

[devblackops]

Checklist

• [ ] All new jobs, commands, executors, parameters have descriptions
• [ ] Examples have been added for any significant new features
• [ ] README has been updated, if necessary

Motivation, issues

Fixes #137

Description

Support expanding env vars on the extra_build_args parameter.

[orb-publisher]

Your development orb has been published. It will expire in 30 days. You can preview what this will look like on the CircleCI Orb Registry at the following link: https://circleci.com/developer/orbs/orb/circleci/docker?version=dev:bf0d47286c28bee276838aaab16a551444eaec23

[orb-publisher]

Your development orb has been published. It will expire in 30 days. You can preview what this will look like on the CircleCI Orb Registry at the following link: https://circleci.com/developer/orbs/orb/circleci/docker?version=dev:bf0d47286c28bee276838aaab16a551444eaec23

[orb-publisher]

Your development orb has been published. It will expire in 30 days. You can preview what this will look like on the CircleCI Orb Registry at the following link: https://circleci.com/developer/orbs/orb/circleci/docker?version=dev:36d33fefcaccfad0d11861c2e58b5d00999550b0

[orb-publisher]

Your development orb has been published. It will expire in 30 days. You can preview what this will look like on the CircleCI Orb Registry at the following link: https://circleci.com/developer/orbs/orb/circleci/docker?version=dev:ce0c7f2e44bc8b515bc9c95e22af1d7e5f159c13