WIP Extend Dockerfile based build strategies with target build stage

[SaschaSchwarze0]

Changes

I recently stumbled over a single Dockerfile that combined multiple output images and one had to run docker build --target STAGE to select which one should actually be built. And that was not possible with our sample build strategies because we had no parameter for the target stage. Adding this.

e2e will be added when shipwright-io/sample-go#16 is merged

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Set a kind label on this PR
• Release notes block has been filled in, or marked NONE

Release Notes

The Dockerfile-based sample build strategies for BuildAh, BuildKit and Kaniko now have a target parameter to define the stage to be built

[openshift-ci]

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