docs: add serviceaccount driver-opt for k8s driver

Signed-off-by: Justin Chadwell <me@jedevc.com>

docs/reference/buildx_create.md

@@ -163,6 +163,7 @@ No driver options.
 - `requests.memory` - Sets the request memory value specified in bytes or with a valid suffix. Example `requests.memory=500Mi`, `requests.memory=4G`
 - `limits.cpu` - Sets the limit CPU value specified in units of Kubernetes CPU. Example `limits.cpu=100m`, `limits.cpu=2`
 - `limits.memory` - Sets the limit memory value specified in bytes or with a valid suffix. Example `limits.memory=500Mi`, `limits.memory=4G`
+- `serviceaccount` - Sets the created pod's service account. Example `serviceaccount=example-sa`
 - `"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 tolera>tions. 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`. 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.