fix: validator version displayed in validator report summary

[jcpitre]

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.

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[davidgamez]

LGTM