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: c8dc2b156a079ce03db8f579094b9643632661a8
Component: engine
9a730a715e