Share local Docker images between jobs when running the CI pipeline in a PR

[alessfg]

Proposed changes

See PR title.

Checklist

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

• [ ] I have read the CONTRIBUTING document
• [ ] I have run ./update.sh and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• [ ] I have tested that the NGINX Unprivileged Docker images build and run correctly on all supported architectures on an unprivileged environment (check out the README for more details)
• [ ] I have updated any relevant documentation (README)