build single image

[pdabelf5]

Proposed changes

Workflow to build and push a single image via a make target.

Options:

inputs.target    # make target to call
inputs.prefix    # prefix to use in GCR, e.g. nginx-ic
inputs.tag       # tag to use in GCR, e.g. 3.7.0-SNAPSHOT
inputs.branch    # branch to checkout for build
inputs.plus_repo # plus repo to install from

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork