feat: Use jib for dataflow flex template

[irvifa]

Fixes #issue

It's a good idea to open an issue first for discussion.

• [x] I have followed Sample Format Guide
• [x] pom.xml parent set to latest shared-configuration
• [x] Appropriate changes to README are included in PR
• [x] API's need to be enabled to test (tell us)
• [x] Environment Variables need to be set (ask us to set them)
• [x] Tests pass: mvn clean verify required
• [x] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] Please merge this PR for me once it is approved.