Add UI for Fleet Sandbox to download prepackaged installers (#6721)

docs/Contributing/Testing.md

@@ -409,7 +409,8 @@ awslocal kinesis get-records --shard-iterator AAAAAAAAAAERtiUrWGI0sq99TtpKnmDu6h
 
 Pre-built installers are kept in a blob storage like AWS S3. As part of your your local development there's a [MinIO](https://min.io/) instance running on http://localhost:9000. To test the pre-built installers functionality locally:
 
-1. Build the installers you want using `fleetctl package`.
+1. Build the installers you want using `fleetctl package`. Be sure to include the `--insecure` flag
+   for local testing.
 2. Use the [installerstore](../../tools/installerstore/README.md) tool to upload them to your MinIO instance.
 3. Configure your fleet server setting `FLEET_PACKAGING_GLOBAL_ENROLL_SECRET` to match your global enroll secret.
 4. Set `FLEET_DEMO=1`, as the endpoint to retrieve the installer is only available in the sandbox.
@@ -418,4 +419,7 @@ Pre-built installers are kept in a blob storage like AWS S3. As part of your you
 FLEET_DEMO=1 FLEET_PACKAGING_GLOBAL_ENROLL_SECRET=xyz  ./build/fleet serve --dev 
 ```
 
+Be sure to replace the `FLEET_PACKAGING_GLOBAL_ENROLL_SECRET` value above with the global enroll
+secret from the `fleetctl package` command used to build the installers. 
+
 MinIO also offers a web interface at http://localhost:9001. Credentials are `minio` / `minio123!`.