build(s3-cache): upload_parallelism opt

Signed-off-by: David Karlsson <[email protected]>

Author: dvdksn

content/manuals/build/cache/backends/s3.md

@@ -37,6 +37,7 @@ The following table describes the available CSV parameters that you can pass to
 | `name`              | `cache-to`,`cache-from` | String      |         | Name of the cache image.                                   |
 | `endpoint_url`      | `cache-to`,`cache-from` | String      |         | Endpoint of the S3 bucket.                                 |
 | `blobs_prefix`      | `cache-to`,`cache-from` | String      |         | Prefix to prepend to blob filenames.                       |
+| `upload_parallelism`| `cache-to`              | Integer     | `4`     | Number of parallel layer uploads.                          |
 | `manifests_prefix`  | `cache-to`,`cache-from` | String      |         | Prefix to prepend on manifest filenames.                   |
 | `use_path_style`    | `cache-to`,`cache-from` | Boolean     | `false` | When `true`, uses `bucket` in the URL instead of hostname. |
 | `access_key_id`     | `cache-to`,`cache-from` | String      |         | See [authentication][1].                                   |