Summary:

Fixes #1620
the problem was that we were using the same Docker image for staging and prod.
Configured so the staging Docker image has a different name than the production one.

Expected behavior:
After executing the web_svc_pr.yml github action, tested that:

• When running the validator at https://gtfs-validator.mobilitydata.org/ the version in the result html file is 4.2.0
• When running at https://gtfs-validator-staging.mobilitydata.org/ the version is 4.2.1-SNAPSHOT.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)