Provide a "dry run" flag to the build-push-images workflow

[sjd78]

Add a "dry run" flag to the build-push-images workflow to enable image build but skip the push to repo. This feature would allow reuse of the full build-push-images workflow in as a test to make sure that image builds will work with a change in a given PR.

Ref: konveyor/tackle2-ui#1907 (comment)

This is in support of the workflow created in konveyor/tackle2-ui#1907 that implements basic CI for Dockerfile and package-lock.json file changes to go along with dependabot docker image tracking konveyor/tackle2-ui#1883.

By adding the flag, the test-image-build job of the tackle2-ui workflow https://github.com/konveyor/tackle2-ui/blob/f501fb80f50830a9437db27460ccd315329e793e/.github/workflows/ci-image-build.yml#L69-L105 could be replaced with a call to build-push-images and ensure the test build and the actual build-push are using the same steps.

[konveyor-ci-bot]

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