Set BUNDLE_FROZEN to true

Alejandro Rodríguezrequested to merge
freeze-bundle into main

To follow rubygems' security adisory https://github.com/rubygems/rubygems.org/security/advisories/GHSA-hccv-rwq6-vh79:

We go with this option over --deployment to match what we did on gitlab-org/gitlab (gitlab!87053 (merged))

Closes #562 (closed)

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