add a do-not-submit workflow

[devoncarew]

• add a do-not-submit workflow

This workflow will hopefully eliminate accidental merges from the github UI.

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[copybara-service[bot]]

Thank you for your contribution. This project uses Gerrit for code reviews. Your pull request has automatically been converted into a code review at:

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Please wait for a developer to review your code review at the above link. See CONTRIBUTING.md to learn how to upload changes to Gerrit directly. You can speed up the review if you sign into Gerrit and manually add a reviewer that has recently worked on the relevant code and they will help you. You can also push additional commits to this pull request to update the code review.

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https://dart-review.googlesource.com/c/dart-docker/+/303401 has been updated with the latest commits from this pull request.

[copybara-service[bot]]

https://dart-review.googlesource.com/c/dart-docker/+/303401 has been updated with the latest commits from this pull request.