Flag quoted arguments in ARG, ENV, and LABEL as an error

[rcjsuen]

The following two Dockerfiles will pass the validator just fine.

FROM scratch
ENV "a.b.c=xyz"

FROM scratch
ENV "a.b.c=xyz

However, Docker will refuse to build it. If possible, we should try to detect this case and flag an error as well.

$ docker build .
Sending build context to Docker daemon  1.693MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : ENV "a.b.c=xyz"
failed to process "\"a.b.c": unexpected end of statement while looking for matching double-quote

$ docker build .
Sending build context to Docker daemon  1.693MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : ENV "a.b.c=xyz
failed to process "\"a.b.c": unexpected end of statement while looking for matching double-quote

[rcjsuen]

These are also invalid for ARG and LABEL.