kubernetes: replace deprecated seccomp annotations with securityContext

Kubernetes added the official `securityContext.seccompProfile` support in Kubernetes 1.19.
Seccomp is still disabled by default.

The legacy `container.seccomp.security.alpha.kubernetes.io/<PODNAME>` annotation has been deprecated and will be unsupported in Kubernetes 1.25.
https://kubernetes.io/docs/tutorials/security/seccomp/

A test cluster can be created with the following minikube command:
```
minikube start --feature-gates SeccompDefault=true --extra-config kubelet.seccomp-default=true
```

Related to moby/buildkit PR 2782

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

docs/reference/buildx_create.md

@@ -141,7 +141,7 @@ Passes additional driver-specific options. Details for each driver:
   - `limits.memory` - Sets the limit memory value specified in bytes or with a valid suffix. Example `limits.memory=500Mi`, `limits.memory=4G`
   - `nodeselector="label1=value1,label2=value2"` - Sets the kv of `Pod` nodeSelector. No Defaults. Example `nodeselector=kubernetes.io/arch=arm64`
   - `tolerations="key=foo,value=bar;key=foo2,operator=exists;key=foo3,effect=NoSchedule"` - Sets the `Pod` tolerations. Accepts the same values as the kube manifest tolerations. Key-value pairs are separated by `,`, tolerations are separated by `;`. No Defaults. Example `tolerations=operator=exists`
-  - `rootless=(true|false)` - Run the container as a non-root user without `securityContext.privileged`. [Using Ubuntu host kernel is recommended](https://github.com/moby/buildkit/blob/master/docs/rootless.md). Defaults to false.
+  - `rootless=(true|false)` - Run the container as a non-root user without `securityContext.privileged`. Needs Kubernetes 1.19 or later. [Using Ubuntu host kernel is recommended](https://github.com/moby/buildkit/blob/master/docs/rootless.md). Defaults to false.
   - `loadbalance=(sticky|random)` - Load-balancing strategy. If set to "sticky", the pod is chosen using the hash of the context path. Defaults to "sticky"
   - `qemu.install=(true|false)` - Install QEMU emulation for multi platforms support.
   - `qemu.image=IMAGE` - Sets the QEMU emulation image. Defaults to `tonistiigi/binfmt:latest`

driver/kubernetes/manifest/manifest.go

@@ -204,12 +204,15 @@ func toRootless(d *appsv1.Deployment) error {
 		d.Spec.Template.Spec.Containers[0].Args,
 		"--oci-worker-no-process-sandbox",
 	)
-	d.Spec.Template.Spec.Containers[0].SecurityContext = nil
+	d.Spec.Template.Spec.Containers[0].SecurityContext = &corev1.SecurityContext{
+		SeccompProfile: &corev1.SeccompProfile{
+			Type: corev1.SeccompProfileTypeUnconfined,
+		},
+	}
 	if d.Spec.Template.ObjectMeta.Annotations == nil {
-		d.Spec.Template.ObjectMeta.Annotations = make(map[string]string, 2)
+		d.Spec.Template.ObjectMeta.Annotations = make(map[string]string, 1)
 	}
 	d.Spec.Template.ObjectMeta.Annotations["container.apparmor.security.beta.kubernetes.io/"+containerName] = "unconfined"
-	d.Spec.Template.ObjectMeta.Annotations["container.seccomp.security.alpha.kubernetes.io/"+containerName] = "unconfined"
 	return nil
 }