Add requirements for Docker image in examples

[pleshakov]

Proposed changes

Problems:

• For security, we want to use Docker images we can trust -- either managed by NGINX or F5 or an official Docker images.
• We want contributors to be aware of that requirement.

Solution:

• Add requirements to the developer docs.

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