Add template-only-cd for integration testing

[rocketnova]

Ticket

• Related to Add infra integration test and CD #7

Changes

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• Adds .github/workflows/template-only-cd.yml

Context for reviewers

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This PR builds on top of #29. It adds automated integration testing with the Platform infrastructure template. When PRs are merged into the main branch of this repo (template-application-rails), the changes will automatically be committed to the main branch of a separate test repo: https://github.com/navapbc/platform-test-rails.

This follows the same pattern that is currently used for the nextjs application template and the flask application template.

Testing

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