feat: add forgejo actions yaml to auto build and push a container image

.forgejo/workflows/build.yaml

@@ -0,0 +1,21 @@
+on:
+  push:
+    branches:
+      - main
+jobs:
+  build:
+    runs-on: [ nix ]
+    steps:
+    - uses: actions/checkout@v4
+    - name: Nix build
+      run: nix build --extra-experimental-features "flakes nix-command" .#docker-image
+    - name: Setup skopeo
+      run: curl -o /etc/containers/policy.json --create-dirs https://raw.githubusercontent.com/containers/skopeo/refs/heads/main/default-policy.json
+    - name: Push Image
+      run: |
+       skopeo copy --dest-password "$UPLOAD_TOKEN" --dest-username "$USERNAME" "docker-archive://$(realpath result)" "docker://$IMG_URL:${{ github.sha }}" && \
+       skopeo copy --dest-password "$UPLOAD_TOKEN" --dest-username "$USERNAME" "docker-archive://$(realpath result)" "docker://$IMG_URL"
+      env: 
+        USERNAME: tamipes
+        UPLOAD_TOKEN: ${{ secrets.UPLOAD_TOKEN }}
+        IMG_URL: git.tami.moe/tamipes/minecraft-ingress