Add standalone mode docs and ci job for testing

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

.github/workflows/ci.yml

@@ -389,3 +389,48 @@ jobs:
         uses: docker/build-push-action@master
         with:
           context: .
+
+  standalone-kubernetes:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Uninstall moby
+        run: |
+          sudo apt-get purge -y moby-engine moby-cli moby-buildx
+      -
+        name: Setup k8s cluster
+        run: |
+          set -x
+          sudo curl -fsSLo /usr/share/keyrings/kubernetes-archive-keyring.gpg https://packages.cloud.google.com/apt/doc/apt-key.gpg
+          echo "deb [signed-by=/usr/share/keyrings/kubernetes-archive-keyring.gpg] https://apt.kubernetes.io/ kubernetes-xenial main" | sudo tee /etc/apt/sources.list.d/kubernetes.list
+          sudo apt-get update
+          sudo apt-get install -y kubelet kubeadm kubectl
+          sudo swapoff -a
+          sudo kubeadm init --cri-socket /run/containerd/containerd.sock
+          mkdir -p $HOME/.kube/
+          sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
+          sudo chown $USER $HOME/.kube/config
+          kubectl taint nodes --all node-role.kubernetes.io/master-
+          kubectl apply -f https://docs.projectcalico.org/manifests/calico.yaml
+          kubectl wait --for=condition=ready --timeout=30s node --all
+          kubectl get nodes -o wide
+      -
+        name: Create Dockerfile
+        run: |
+          cat > ./Dockerfile <<EOL
+          FROM alpine
+          RUN echo hello
+          EOL
+      -
+        name: Set up Docker Buildx
+        uses: ./
+        with:
+          standalone: true
+          driver: kubernetes
+      -
+        name: Build
+        run: |
+          buildx build .

README.md

@@ -24,6 +24,7 @@ ___
   * [BuildKit daemon configuration](#buildkit-daemon-configuration)
     * [Registry mirror](#registry-mirror)
     * [Max parallelism](#max-parallelism)
+  * [Standalone mode](#standalone-mode)
 * [Customizing](#customizing)
   * [inputs](#inputs)
   * [outputs](#outputs)
@@ -180,6 +181,34 @@ jobs:
           config: .github/buildkitd.toml
 ```
 
+### Standalone mode
+
+If you don't have the Docker engine and CLI installed on your GitHub Runner,
+you can use buildx in standalone mode. This can be useful if you want to use
+the `kubernetes` driver in your self-hosted runner:
+
+```yaml
+name: ci
+
+on:
+  push:
+
+jobs:
+  buildx:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+        with:
+          standalone: true
+          driver: kubernetes
+      -
+        name: Build
+        run: |
+          buildx build .
+```
+
 ## Customizing
 
 ### inputs