Add runAsHostUser docs (#438)

* Add runAsHostUser docs

* Update docs/03-github/04-builder.mdx

---------

Co-authored-by: Gabriel Le Breton <[email protected]>

docs/03-github/04-builder.mdx

@@ -398,6 +398,17 @@ This is useful if your manifest has a dependency on a private GitHub repo.
 
 _**required:** `false`_ _**default:** `""`_
 
+#### runAsHostUser
+
+Run the container as the host user based on the file permissions of the cloned project that gets
+mounted to the container. This is useful if you are using a self-hosted runner and need the
+generated files from the container to be owned by the same user as the runner host. This solves most
+permission errors without requiring the runner agent to run as root. This option shouldn't be needed
+on Github Hosted runners as the systems are automatically wiped each run so we default to `false`.
+Self-Hosted runners generally will want to set this to `true`.
+
+_**required:** `false`_ _**default:** `false`_
+
 #### chownFilesTo
 
 User and optionally group (user or user:group or uid:gid) to give ownership of the resulting build