How to Properly Implement CI/CD with Merging Strategies?

It looks like there's a misunderstanding here. The point of "build once" is about promoting a single finalized image through different environments, like from staging to production. This ensures no discrepancies arise between builds. Building directly from a PR might not be the best practice since the code there could still change. It's usually better to build on merges to the master branch and tag those images distinctly for future references.

Using a trunk-based strategy with tagging will definitely help you here. Just ensure you maintain a linear history to streamline the process.

You might be a bit off on the "Build once" concept. When you're using trunk-based development, you should build from the main branch after merging pulls to create one final image. Think of it this way: every time there’s a change to main, that’s when you build and deploy the new artifact. So, essentially, merging PR B should trigger a build with both A and B changes. Maybe consider elaborating on your branching strategy a bit more too.

There are different approaches to handling this. What I typically do is adopt semantic versioning. Instead of tagging every merge, just tag the main branch whenever you're ready to deploy, like 1.10.0. You can also build the Docker image on merges and tag it as "main" or "latest." This helps in thinking of your project like it's ready for public release. And if you're building from feature branches, you might just want to do it for validation purposes. If that’s the case, tagging them by the commit SHA or PR ID could be a good idea.