Implement Dockerfile template verification for internal servicing validation

[NikolaMilosavljevic]

This issue was brought up with #5928 (comment)

Our templates support multiple scenarios for dockerfile generation, single and multi-stage are just one example. With internal testing work (#5928) it becomes critical to ensure that no dockerfile template change is ever regressing any of the supported dockerfile generation scenarios, i.e. internal testing.

Verification should include generation of templates and appropriate pipeline run to validate build and test.

Some options:

• Manual verification of internal testing scenario, as a requirement for any template change
• Automated verification of internal testing scenario
• Automated verification of all supported scenarios