docs: Refactor ServiceConfig to be more informative

.gitignore

@@ -131,4 +131,4 @@ lerna-debug.log
 # contains several libraries that get shipped
 yarn.lock
 package.json
-package-lock.json
+package-lock.json

docs/docs/api-reference/starlark-reference/image-spec.md

@@ -0,0 +1,43 @@
+---
+title: ImageSpec
+sidebar_label: ImageSpec
+---
+
+The `ImageSpec` object constructor allows for providing detailed information about a container image for use in the [`ServiceConfig.image`](./service-config.md) property. It is most commonly used when pulling an image from a non-Dockerhub container repository.
+
+Signature
+---------
+
+```
+ImageSpec(
+    name, 
+    registry = "http://hub.docker.com", 
+    username = None, 
+    password = None,
+)
+```
+
+| Property | Description |
+| --- | --- |
+| **name**<br/>_string_ | The name of the container image that should be used. |
+| **registry**<br/>_string_ | The registry that the container should be pulled from. |
+| **username**<br/>_string_ | The username that will be used for connecting to the registry when pulling the image. |
+| **password**<br/>_string_ | The password that will be used for connecting to the registry when pulling the image. |
+
+Examples
+--------
+Starting a service called `my-service` from `my.registry.io` using a custom username and password:
+
+```python
+plan.add_service(
+    name = "my-service",
+    config = ServiceConfig(
+        image = ImageSpec(
+            name = "my.registry.io/some-org/some-image",
+            registry = "http://my.registry.io/",
+            username = "registry-user",
+            password = "SomePassword1!",
+        )
+    )
+)
+```