@rodgco did you create a new branch when pushing the amended GitHub workflow triggers?
As for only building and pushing the images for a PR that is approved and merged - you could do this, yes, but we wouldn’t recommend it. Having your CI build a Docker tag for your PRs (even before they are approved) means that anyone (developers or QA) can easily QA your changes by running the image created in the CI e.g. in a test/staging environment.