Update docs

docs/tarball.md

@@ -25,7 +25,7 @@ docker run --rm my-repository:latest
 ## oci_tarball
 
 <pre>
-oci_tarball(<a href="#oci_tarball-name">name</a>, <a href="#oci_tarball-image">image</a>, <a href="#oci_tarball-repo_tags">repo_tags</a>)
+oci_tarball(<a href="#oci_tarball-name">name</a>, <a href="#oci_tarball-format">format</a>, <a href="#oci_tarball-image">image</a>, <a href="#oci_tarball-repo_tags">repo_tags</a>)
 </pre>
 
 Creates tarball from OCI layouts that can be loaded into docker daemon without needing to publish the image first.
@@ -39,6 +39,7 @@ Passing anything other than oci_image to the image attribute will lead to build
 | Name  | Description | Type | Mandatory | Default |
 | :------------- | :------------- | :------------- | :------------- | :------------- |
 | <a id="oci_tarball-name"></a>name |  A unique name for this target.   | <a href="https://bazel.build/docs/build-ref.html#name">Name</a> | required |  |
+| <a id="oci_tarball-format"></a>format |  Format of image to generate. Options are: docker, oci. Currently, when the input image is an image_index, only oci is supported, and when the input image is an image, only docker is supported. Conversions between formats may be supported in the future.   | String | optional | "docker" |
 | <a id="oci_tarball-image"></a>image |  Label of a directory containing an OCI layout, typically <code>oci_image</code>   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | required |  |
 | <a id="oci_tarball-repo_tags"></a>repo_tags |  a file containing repo_tags, one per line.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | required |  |