This fix tries to address the issue raised in #23498 to allow unset
`--entrypoint` in `docker run` or `docker create`.
This fix checks the flag `--entrypoint` and, in case `--entrypoint=` (`""`)
is passed, unset the Entrypoint during the container run.
Additional integration tests have been created to cover changes in this fix.
This fix fixes#23498.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>