Switch CI to use buildah, following hswro_org pattern

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/docker.yaml

@@ -6,17 +6,12 @@ on:
 jobs:
   build:
     runs-on: docker
+    container:
+      image: quay.io/buildah/stable
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-
       - name: Build and push Docker image
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          push: true
-          tags: ${{ gitea.server_url }}/${{ gitea.repository }}:latest
-          username: ${{ gitea.actor }}
-          password: ${{ secrets.FORGEJO_TOKEN }}
+        run: |
+          buildah build -t kalendasz:latest
+          buildah push --creds ${{ gitea.actor }}:${{ secrets.FORGEJO_TOKEN }} kalendasz:latest "docker://git.hswro.org/kosma/kalendasz:latest"