Fix default build mechanism priority for jib

[grayside]

Default build priority doesn't cover all the supported build methods, and incorrectly prioritizes Dockerfiles over Jib for Java.

Current Practice

• README command
• Dockerfile
• Jib

Preferred

• README command
• Jib
• Dockerfile

Notes on the preferred ordering:

• Jib is the primary recommendation for building Java containers in Cloud Run. Jib may be configured in a project to use a Dockerfile to create a base image.

This raises a secondary problem: what if it's a Java project that wants to build from a Dockerfile, not use jib?

This issue affects two categories in my review of the repository:

• Correctness: Is the implemented business logic correct?
• Scope: Does the tool have the breadth of features and flexibility to support reasonable testing needs?

[SaketramDurbha]

Draft implementation here: https://github.com/SaketramDurbha/serverless-sample-tester/tree/i7 .

With regards to users wanting to build a java sample without jib, I think we can force them to use a method outside of reasonable defaults to specify their build and deploy commands (i.e. Cloud Build configuration, when that's implemented). Or we can implement a --no-jib flag as well.