ENV declaration does not support regex in string

[matthewh86]

Summary

If an ENV declaration has some arbitrary regex it fails linter.

Expected behaviour

ENV should support setting arbitrary strings with control characters in them.

Actual behaviour

Line 3: -> ENV LATEST_GRADLE_REGEX=".+distributions\/gradle-(6.([7-9]|([1-9][0-9]){2,})\.*[0-9]*)-all\.zip\.sha256.*"
ERROR: Invalid parameters for command.. 
Reference -> https://docs.docker.com/engine/reference/builder/

Steps to reproduce

Dockerfile:

FROM alpine:latest

ENV LATEST_GRADLE_REGEX=".+distributions\/gradle-(6.([7-9]|([1-9][0-9]){2,})\.*[0-9]*)-all\.zip\.sha256.*"