doc(buildkitd.toml.md): Add mirror configuration

Signed-off-by: Hans van den Bogert <hansbogert@gmail.com>

docs/buildkitd.toml.md

@@ -90,11 +90,15 @@ insecure-entitlements = [ "network.host", "security.insecure" ]
 
 # registry configures a new Docker register used for cache import or output.
 [registry."docker.io"]
-  mirrors = ["hub.docker.io"]
+  mirrors = ["yourmirror.local:5000"]
   http = true
   insecure = true
   ca=["/etc/config/myca.pem"]
   [[registry."docker.io".keypair]]
     key="/etc/config/key.pem"
     cert="/etc/config/cert.pem"
+    
+# optionally mirror configuration can be done by defining it as a registry.
+[registry."yourmirror.local:5000"]
+  http = true
 ```