Document ASPIRE_ENVIRONMENT for AppHost

src/frontend/src/content/docs/app-host/configuration.mdx

@@ -80,8 +80,13 @@ In TypeScript AppHosts, profiles live in `aspire.config.json`:
 | `ASPIRE_ALLOW_UNSECURED_TRANSPORT`       | `false`       | Allows communication with the AppHost without https. `ASPNETCORE_URLS` (dashboard address) and `ASPIRE_RESOURCE_SERVICE_ENDPOINT_URL` (AppHost resource service address) must be secured with HTTPS unless true. |
 | `ASPIRE_CONTAINER_RUNTIME`               | `docker`      | Allows the user of alternative container runtimes for resources backed by containers. Possible values are `docker` (default) or `podman`.                                                                        |
 | `ASPIRE_DCP_USE_DEVELOPER_CERTIFICATE`   | `false`       | When set to `true`, Aspire uses its trusted developer certificate to secure the internal DCP server instead of an ephemeral certificate generated by DCP. This can help avoid certificate trust issues when the dev cert is already trusted. If no trusted developer certificate is found, Aspire falls back to the DCP-generated ephemeral certificate. Only supported on Windows. For more information, see [Certificate configuration](/app-host/certificate-configuration/). |
+| `ASPIRE_ENVIRONMENT`                     | `null`        | Configures the AppHost environment when no higher-priority environment source is set. If no environment is configured, the AppHost uses `Production`.                                                            |
 | `ASPIRE_VERSION_CHECK_DISABLED`          | `false`       | When set to `true`, Aspire doesn't check for newer versions on startup.                                                                                                                                          |
 
+## AppHost environment
+
+Use `ASPIRE_ENVIRONMENT` to set the environment name used by the AppHost while it evaluates the application model. Precedence is `--environment`, `DOTNET_ENVIRONMENT`, `ASPIRE_ENVIRONMENT`, then `Production`. This doesn't configure the dashboard's `ASPNETCORE_ENVIRONMENT` or automatically flow to child resources; set framework-specific variables on resources as needed. For details, see [Aspire environments](/deployment/environments/).
+
 ## Version update notifications
 
 When an Aspire app starts, it checks if a newer version of Aspire is available on NuGet. If a new version is found, a notification appears in the dashboard with the latest version number, [a link to upgrade instructions](https://aka.ms/dotnet/aspire/update-latest), and button to ignore that version in the future.