Refine CI triggers for building OCI

[Zerpet]

Proposed Changes

• Trigger OCI builds only on code changes
• Build OCI only on commits to main

Right now, we are building a dev OCI on any type of changes. For example, changes to Selenim tests would
trigger a build of the OCI. This doesn't make sense because changes to tests do not alter RabbitMQ functionality.
This PR changes the triggers so that CI builds the OCI when there are changes to RabbitMQ code or its dependencies.

Types of Changes

What types of changes does your code introduce to this project?
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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

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• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

[Zerpet]

@mkuratczyk I changed the workflow to trigger only on commits to main. I'm not sure whether that's desirable, or you'd prefer to build OCI for WIP branches as well. I don't have a strong opinion.

[mkuratczyk]

Comparing the behaviour/performance of WIP branches vs main is one of the main uses cases for me, so I'd rather have the images built on branches as well.

[Zerpet]

Dropped the commit that filters the branch only to main. Now commits that match the path filter in any branch will trigger the dev OCI build.