add CI workflow to build and push MkDocs Docker image

.gitea/workflows/build-and-push-mkdocs-docker-image.yml

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+name: Build and Push MkDocs Docker Image
+
+on:
+    push:
+        branches:
+            - main
+            - master
+        paths:
+            - 'Dockerfile'
+            - 'user/**'
+            - 'admin/**'
+            - '.gitea/workflows/**'
+    workflow_dispatch:
+
+jobs:
+    build-and-push:
+        runs-on: cth-ubuntu-latest
+
+        env:
+            REGISTRY: ${{ vars.REGISTRY }}
+            IMAGE: ${{ vars.IMAGE }}
+            IMAGE_NAME: ${{ env.REGISTRY }}/${{ IMAGE }}:${{ github.sha }}
+            KUBE_CA: ${{ vars.KUBE_CA }}
+            KUBE_SERVER: ${{ vars.KUBE_SERVER }}
+            REGISTRY_USERNAME: ${{ vars.REGISTRY_USERNAME }}
+            NAMESPACE: ${{ vars.NAMESPACE }}
+            DEPLOYMENT_NAME: ${{ vars.DEPLOYMENT_NAME }}
+            CONTAINER: ${{ vars.CONTAINER }}
+
+        steps:
+            -   name: Checkout code
+                uses: https://github.com/actions/checkout@v4
+
+            -   name: Set up Docker Buildx
+                uses: https://github.com/docker/setup-buildx-action@v3
+
+            -   name: Log in to Docker registry
+                uses: https://github.com/docker/login-action@v3
+                with:
+                    registry: ${{ env.REGISTRY }}
+                    username: ${{ env.REGISTRY_USERNAME }}
+                    password: ${{ secrets.REGISTRY_PASSWORD }}
+
+            -   name: Build and push
+                id: build-push
+                uses: https://github.com/docker/build-push-action@v5
+                with:
+                    context: .
+                    file: documentation.Dockerfile
+                    push: true
+                    tags: '${{ env.IMAGE_NAME }}'
+
+            -   name: Install kubectl
+                run: |
+                    curl -LO "https://dl.k8s.io/release/v1.31.0/bin/linux/amd64/kubectl"
+                    chmod +x ./kubectl
+                    sudo mv ./kubectl /usr/local/bin/kubectl
+
+            -   name: Set up kubeconfig
+                env:
+                    KUBE_SERVER: ${{ env.KUBE_SERVER }}      # e.g. https://<API_ENDPOINT>
+                    KUBE_CA: ${{ env.KUBE_CA }}              # base64 encoded, or fetched via kubectl config view
+                    KUBE_TOKEN: ${{ secrets.KUBE_TOKEN }}        # Token generated via 'kubectl create token'
+                run: |
+                    mkdir -p ~/.kube
+                    cat <<EOF > ~/.kube/config
+                    apiVersion: v1
+                    kind: Config
+                    clusters:
+                      - name: default-cluster
+                        cluster:
+                          server: "$KUBE_SERVER"
+                          certificate-authority-data: "$KUBE_CA"
+                    contexts:
+                      - name: default-context
+                        context:
+                          cluster: default-cluster
+                          user: gitea-actions
+                    current-context: default-context
+                    users:
+                      - name: gitea-actions
+                        user:
+                          token: "$KUBE_TOKEN"
+                    EOF
+
+            -   name: Patch Deployment Image
+                env:
+                    NAMESPACE: ${{ env.NAMESPACE }}
+                    DEPLOYMENT: ${{ env.DEPLOYMENT_NAME }}
+                    CONTAINER: ${{ env.CONTAINER }}
+                run: |
+                    echo "deploy new tag: $TAG_IMAGE"
+                    env
+                    kubectl -n ${NAMESPACE} set image deployment/${DEPLOYMENT} ${CONTAINER}=${IMAGE_NAME}