This fix tries to address the issue raised in #20083 where
comment is not supported in `.dockerignore`.
This fix updated the processing of `.dockerignore` so that any
lines starting with `#` are ignored, which is similiar to the
behavior of `.gitignore`.
Related documentation has been updated.
Additional tests have been added to cover the changes.
This fix fixes#20083.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
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