Add distribution release/version tags to Docker images

[alessfg]

Proposed changes

This PR aims to:
a) Add distribution release/version tags to new Docker images (similarly to what was done in nginx/docker-nginx@beac75e in the upstream Docker NGINX repo)
b) Simplify the GitHub actions workflows to avoid duplicating code when pushing new Docker images to the various registries supported by this image.

Checklist

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

• I have read the CONTRIBUTING document.
• I have tested that the NGINX Unprivileged Docker images build correctly on all supported platforms (check out the README for more details).
• I have deployed the NGINX Unprivileged Docker images on an unprivileged environment and checked that they run correctly.
• I have updated any relevant documentation (README)