Docker file 1.7 syntax support

## Description

https://www.docker.com/blog/new-dockerfile-capabilities-v1-7-0/

Need to support new syntax for substituting env vars when evaling docker file, otherwise we can compute wrong base image for example

Start with the linked Docker blog post to identify the Dockerfile 1.7 environment-variable substitution syntax, then locate Chalk's Dockerfile evaluation entry point; the issue names no files or tests. Done means evaluating affected Dockerfiles computes the correct base image rather than the wrong one.

Written by the indexing model from the issue text.