Login Dockerhub step is failing

[ryanshepps]

On each pull request, the "Login to DockerHub" step is failing.

Example: https://github.com/UoGSOCIS/socis_website/runs/6406379102?check_suite_focus=true

[SoorajModi]

This issue is caused by environment variables in our CI files not being accessible for forks of this repo. The solution is to modify the staging.yml pipeline to only run for branches in this repo, or not run at all and trigger it manually.