add docker builder using kubernetes natively

infrastructure/gitea/README.md

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+# Using gitea actions
+The actions deployment allows to use gitea actions from repositories within this instance.
+
+### Building docker images
+Docker builds use the kubernetes runner to build the images. For this to work, the pipeline needs to be able to access the kube-api. A service-account is created for this purpose.
+
+To use the correct docker builder use the following action
+```yaml
+    ...
+
+    - name: Create Kubeconfig
+      run: |
+        mkdir $HOME/.kube
+        echo "${{ secrets.BUILDX_KUBECONFIG }}" > $HOME/.kube/config
+
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+      with:
+        driver: kubernetes
+        driver-opts: |
+          namespace=act-runner
+          qemu.install=true
+
+    ...
+
+    - name: Build and push
+      uses: docker/build-push-action@v5
+      with:
+        context: .
+        <other config>
+```