Dockerhub user/password not availble in PR makes unable to check it could/not be build

For ex. Build #1479 - sdlpal/sdlpal - Travis CI (travis-ci.com)

Which makes one of the main purpose of using CI, making sure a PR can be build, just stops work. I know that dockerhub requesting them, and the need of keeping MOST environment variable secure from PR; could you consider some workaround this usage? Like let user mark some of the environment variable be available to PR?

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