Allow for global ARG substitution between stages

[Rowern]

See: https://github.com/moby/moby/issues/37345#issuecomment-400245466

While doing this I saw that the entrypoint on the image is different than docker (from Dockerfile of testdata/build-context/global-arg/Dockerfile):
Docker:
if [ -z \"$version\" -a \"$version2\" = \"v2\" ]; then echo \"This is correct\"; exit 0; else exit 1; fi Makisu: if [ -z \"$version\" -a \"v2\" = \"v2\" ] ; then echo \"This is correct\" ; exit 0 ; else exit 1 ; fi

Makisu should not do substitution in the CMD and ENTRYPOINT directive (as seen in the list here: https://docs.docker.com/engine/reference/builder/#environment-replacement)

[Rowern]

Integration tests are failing until https://github.com/uber/makisu/pull/323 is merged. I need the fix on ENTRYPOINT PARSING.

[CLAassistant]

All committers have signed the CLA.

[Rowern]

All committers have signed the CLA.

Weird that I had to sign this again...

[Rowern]

EDIT: I'm bad at integration testing  😅