Add startScript option to docs

Author: garethx

packages/homepage/content/docs/3-configuration.md

@@ -32,5 +32,6 @@ support these options:
 | `hardReloadOnChange`     | Whether we should refresh the sandbox page on every change, good for sandboxes with global state    | `true`/`false`                                                                                                     | `false`                                            |
 | `template`               | Which sandbox template to use                                                                       | [see here](https://github.com/codesandbox-app/codesandbox-importers/blob/master/packages/types/index.d.ts#L30-L54) | smart detection, w/ fallback to `create-react-app` |
 | `view`                   | Which view to show first in the preview                                                             | Client: `browser`/`console`/`problems`/`tests`<br />Container: `browser`/`console`/`problems`/`terminal` | `browser`
-| `container`              | The container object contains the configurable port option, for example: `container: { port: 3212}` |
+| `container`              | The container object contains the configurable port and/or startScript option, for example: `container: { port: 3212, startScript: "custom"}` |
 | `port`                   | The main port which the browser window listens to                                                   | 1024 - 65535                                                                                                       | First opened port inside the container.            |
+| `startScript`                   | Explicitly specify the start script used in a container sandbox, overriding the default value                                                 | A String matching a script name defined under `scripts` in `package.json`                                                                                                      | `dev` / `develop` / `serve` / `start` |