Simplify and add additional tests for mp building

[bluesliverx]

Description

This removes the push flag that was never unset when building multiplatform images. It also adds some testing to ensure that things are working correctly in mocking.

Related Issue

Motivation and Context

Simplification and better testing was desired.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.