Add Forgejo Actions pipeline to build and push image

Builds the Containerfile and pushes to git.rosenvold.tech's registry on
push to master.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.forgejo/workflows/build.yml

@@ -0,0 +1,36 @@
+name: Build and push image
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build-and-push:
+    # Needs docker-in-docker: the runner's job container must have access to
+    # a docker socket (e.g. act_runner configured with
+    # -v /var/run/docker.sock:/var/run/docker.sock) for buildx to work.
+    runs-on: docker
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Log in to Forgejo registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.rosenvold.tech
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./Containerfile
+          push: true
+          tags: |
+            git.rosenvold.tech/lucas/rosenvold-core:latest
+            git.rosenvold.tech/lucas/rosenvold-core:${{ github.sha }}