This fix tries to address the issue raised in 26453 where bad syntax
on dockerfile is not checked before building, thus user has to wait
before seeing error in dockerfile.
This fix fixes the issue by evaluating all the instructions and check
syntax before dockerfile is invoked actually.
All existing tests pass.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 824707ea49
Component: cli
473fb49310