fix: use PR number instead of ref_name for Docker image tags

GitHub's github.ref_name returns "421/merge" for PR merge refs,
creating invalid Docker tags like "pr-421/merge". Docker tags
cannot contain forward slashes.

Changed to use github.event.pull_request.number which returns
just the PR number (e.g., "421") for valid tags like "pr-421".

Fixes CI/CD failure in PR #421.

.github/workflows/docker-publish.yml

@@ -101,7 +101,7 @@ jobs:
             type=raw,value=latest,enable={{is_default_branch}}
             type=raw,value=dev,enable=${{ github.ref == 'refs/heads/development' }}
             type=raw,value=beta,enable=${{ github.ref == 'refs/heads/feature/beta-release' }}
-            type=raw,value=pr-${{ github.ref_name }},enable=${{ github.event_name == 'pull_request' }}
+            type=raw,value=pr-${{ github.event.pull_request.number }},enable=${{ github.event_name == 'pull_request' }}
             type=sha,format=short,enable=${{ github.event_name != 'pull_request' }}
 
       - name: Build and push Docker image