update custom docker runtime doc (#520)

src/pages/assets/custom-containers.mdx

@@ -1,41 +1,37 @@
 export const description = 'Customize how services are built into containers'
 
-# Custom containers
+# Custom container runtimes
 
-Nitric builds applications by identifying its entrypoints, which are typically defined in the `nitric.yaml` file as `handlers`. Each entrypoint in a Nitric app is built into its own container using Docker, then deployed to a cloud container runtime such as AWS Lambda, Google CloudRun or Azure Container Apps.
+Nitric builds applications by identifying its entrypoints, which are typically defined in the `nitric.yaml` file as `services`. Each entrypoint in a Nitric app is built into its own container using Docker, then deployed to a cloud container runtime such as AWS Lambda, Google CloudRun or Azure Container Apps.
 
 The Nitric CLI decides how to build those containers based on the programming language used by the entrypoint, for example if the entrypoint is a python file it will be built using Nitric's python dockerfile template. These dockerfile templates are designed with compatibility and ease of use in mind, this makes building applications convenient but may not provide additional dependencies your code relies on the ideal optimization for your application.
 
 If you need to customize the docker container build process to add dependencies, optimize container size or some other reason, you can create a custom dockerfile template to be used by some or all of the entrypoints (services) in your application.
 
-## Specify a dockerfile template
+## Add a new custom runtime
 
-<Note>
-  Custom dockerfiles is currently in preview, you will need to enable this
-  feature in your `nitric.yaml` file.
-</Note>
+Add a new custom runtime in the `runtimes` configuration.
 
-To use a custom dockerfile template update the `handlers` configuration as shown below.
+To use the runtime, simply specify the runtime key per service as shown below.
 
 ```yaml {{ tag: "nitric.yaml" }}
 name: custom-example
-handlers:
+services:
   - match: services/*.ts
-    type: default
-    docker:
-      # All entrypoints in the services directory will be built using this dockerfile
-      file: ./docker/node.dockerfile
-      args: {}
-# Enable preview feature
-preview-features:
-  - dockerfile
+    runtime: 'custom-node' # specify custom runtime
+    start: npm run dev:services $SERVICE_PATH
+runtimes:
+  custom-node:
+    # All services that specify the 'custom-node' runtime will be built using this dockerfile
+    dockerfile: ./docker/node.dockerfile
+    args: {}
 ```
 
 In this example we're specifying that any handlers that match the path `services/*.ts` will use a custom `node.dockerfile` for their dockerfile template.
 
 ## Create a dockerfile template
 
-It's important to note that the custom dockerfile you create needs to act as a template. This can look a bit different to how you might have written dockerfiles in the past, since the same template file will need to be used for all handlers that match the configuration the entrypoint will use a variable which contains the handler's filename.
+It's important to note that the custom dockerfile you create needs to act as a template. This can look a bit different to how you might have written dockerfiles in the past, since the same template file will need to be used for all services that match the configuration the entrypoint will use a variable which contains the service's filename.
 
 Here are some example dockerfiles: