ci: automatically deploy betabooru on push.

Make it so that on every push to the `betabooru` branch, the latest
commit is automatically deployed to https://betabooru.donmai.us.

.github/workflows/deploy-betabooru.yaml

@@ -0,0 +1,46 @@
+# Deploy the site to https://betabooru.donmai.us on every push to the `betabooru` branch.
+
+name: Deploy Betabooru
+
+# https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#permissions
+permissions: read-all
+
+on:
+  # https://docs.github.com/en/actions/reference/events-that-trigger-workflows#workflow_run
+  workflow_dispatch:
+  workflow_run:
+    workflows: ["Docker Build"]
+    types: [completed]
+    branches: [betabooru]
+
+jobs:
+  deploy-betabooru:
+    if: ${{ github.event_name == 'workflow_dispatch' || github.event.workflow_run.conclusion == 'success' }}
+    runs-on: ubuntu-latest
+
+    # https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions#concurrency
+    concurrency: deploy-betabooru
+
+    # The Kubernetes API credentials are kept in the betabooru environment.
+    # https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions#jobsjob_idenvironment
+    environment:
+      name: betabooru
+      url: https://betabooru.donmai.us
+
+    steps:
+    # https://github.com/marketplace/actions/kubectl-tool-installer
+    - name: Install Kubectl
+      uses: azure/setup-kubectl@v1
+      with:
+        version: v1.21.5
+
+    - name: Configure Kubectl
+      run: |
+        mkdir -p ~/.kube
+        echo "$KUBECONFIG_YAML" > ~/.kube/config
+      env:
+        KUBECONFIG_YAML: ${{secrets.KUBECONFIG_YAML}}
+
+    - name: Deploy to Betabooru
+      run: |
+        kubectl rollout restart deploy/betabooru