Docker image rebuilt for dependency update PR

[busunkim96]

PR: #4109

Log: https://source.cloud.google.com/results/invocations/62276164-4adf-42d7-ae21-c96d43abe161/targets

The PR is only updating requirements.txt, but the docker re-build was triggered.

[busunkim96]

It appears to have happened again in #4122

[tmatsuo]

Fixed by #4123