Add Docker workflow

.gitea/workflows/docker.yml

@@ -0,0 +1,53 @@
+name: Docker
+run-name: "Docker: ${{ github.sha }}"
+
+on:
+  push:
+    branches: [main]
+  schedule:
+    - cron: "0 6 * * *"
+  workflow_dispatch:
+
+concurrency:
+  group: docker-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.sha }}
+          fetch-depth: 0
+
+      - name: Check upstream for new opencode image
+        id: upstream
+        run: |
+          TOKEN=$(curl -s "https://ghcr.io/token?service=ghcr.io&scope=repository:anomalyco/opencode:pull" | jq -r .token)
+          DIGEST=$(curl -s -H "Authorization: Bearer $TOKEN" \
+            "https://ghcr.io/v2/anomalyco/opencode/manifests/latest" \
+            -H "Accept: application/vnd.oci.image.index.v1+json" 2>/dev/null | jq -r '.manifests[] | select(.platform.architecture=="amd64" and .platform.os=="linux") | .digest' || echo "")
+          echo "digest=${DIGEST}" >> "$GITHUB_OUTPUT"
+
+      - name: Login to Gitea Package Registry
+        uses: docker/login-action@v3
+        with:
+          registry: gitea.steffenillium.de
+          username: steffen
+          password: ${{ secrets.TOKEN }}
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          platforms: linux/amd64
+          push: true
+          tags: |
+            gitea.steffenillium.de/steffen/opencode-custom:latest
+            gitea.steffenillium.de/steffen/opencode-custom:${{ steps.upstream.outputs.digest || github.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max