Add GitHub Actions workflow for building and pushing Docker images

.gitea/workflows/docker-image.yml

@@ -0,0 +1,56 @@
+name: Build And Push Docker Image
+
+on:
+  push:
+    tags:
+      - "image"
+  workflow_dispatch:
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    env:
+      DOTNET_ROLL_FORWARD: Major
+      REGISTRY: git.beging.de
+      BASE_IMAGE: git.beging.de/troogs/fs-onboarding-server
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Setup .NET SDK
+        uses: actions/setup-dotnet@v5
+        with:
+          dotnet-version: "9.0.x"
+
+      - name: Publish server project
+        run: dotnet publish ./FoodsharingSiegen.Server/FoodsharingSiegen.Server.csproj -c Release -o ./Publish/Server
+
+      - name: Login to Gitea registry
+        run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login "${{ env.REGISTRY }}" -u "${{ secrets.REGISTRY_USERNAME }}" --password-stdin
+
+      - name: Determine next timestamp tag
+        shell: bash
+        run: |
+          TODAY="$(date -u +%Y%m%d)"
+          i=1
+          while docker manifest inspect "${BASE_IMAGE}:${TODAY}-${i}" > /dev/null 2>&1; do
+            i=$((i + 1))
+          done
+
+          DATE_TAG="${TODAY}-${i}"
+          echo "DATE_TAG=${DATE_TAG}" >> "$GITHUB_ENV"
+          echo "Using image tag: ${DATE_TAG}"
+
+      - name: Build docker image
+        run: |
+          docker build \
+            -f ./Docker/dockerfile.server \
+            -t "${{ env.BASE_IMAGE }}:latest" \
+            -t "${{ env.BASE_IMAGE }}:${{ env.DATE_TAG }}" \
+            .
+
+      - name: Push docker images
+        run: |
+          docker push "${{ env.BASE_IMAGE }}:latest"
+          docker push "${{ env.BASE_IMAGE }}:${{ env.DATE_TAG }}"