Update doc for the GKE config section (#747)

Update the documentation for the GKE deployer to mention that the user
can configure the minimum number of replicas for each component, and
that they can specify the base image for the app container.

website/docs.md

@@ -2839,10 +2839,12 @@ listeners.cat = {public_hostname = "cat.com"}
 listeners.hat = {public_hostname = "hat.gg"}
 ```
 
-| Field | Required? | Description                                                                                                            |
-| --- |-----------|------------------------------------------------------------------------------------------------------------------------|
-| regions | yes        | Regions in which the Service Weaver application should be deployed. Note that at least one region should be specified. |
-| listeners | optional  | The application's listener options, e.g., the listeners' public hostnames.                                             |
+| Field       | Required? | Description                                                                                                            |
+|-------------|-----------|------------------------------------------------------------------------------------------------------------------------|
+| regions     | yes       | Regions in which the Service Weaver application should be deployed. Note that at least one region should be specified. |
+| image       | optional      | Base image used to build the application container image. If not specified, `image = ubuntu:rolling`.              |
+| minreplicas | optional      | Minimum number of running pods for each component. If not specified, `minreplicas = 1`.                            |
+| listeners   | optional  | The application's listener options, e.g., the listeners' public hostnames.                                             |
 
 **Note** that by default, your Service Weaver application will be deployed in the
 currently active project using the currently active account; i.e., in the project