chore: testing GitHub Actions with container image

[suztomo]

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Fixes #<issue_number_goes_here> ☕️

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[suztomo]

It worked: https://github.com/googleapis/sdk-platform-java/actions/runs/12505540740/job/34889060553?pr=3482 ("initialize containers" section)

  Digest: sha256:89385dc6f21b4817d605d3f68cc43b515c1fb3ff1e9cb4828322acfaf11729ff
  Status: Downloaded newer image for gcr.io/cloud-devrel-public-resources/java-library-generation:latest
  gcr.io/cloud-devrel-public-resources/java-library-generation:latest