ci: add automatic semver tagging on push to main

Uses anothrNick/github-tag-action (v1.75.0) to bump version and create
a git tag based on #major/#minor/#patch keywords in merge commits.
Runs only on push events to main (i.e. after a PR merge).

Also adds fetch-depth: 0 to checkout so the tag action can see all tags.

Fixes docker/build-push-action SHA: f9a3042 -> f9f3042 (v7.2.0).

.gitea/workflows/docker-build.yaml

@@ -39,7 +39,7 @@ jobs:
             type=raw,value=latest,enable=${{ github.ref == 'refs/heads/main' }}
 
       - name: Build and push
-        uses: docker/build-push-action@f9a3042f7e2789586610d6e8b85c8f03e5195baf # v7
+        uses: docker/build-push-action@f9f3042f7e2789586610d6e8b85c8f03e5195baf # v7.2.0
         with:
           context: .
           push: ${{ github.event_name != 'pull_request' }}