Add schedule to supported signing events in image builder README file (…

…#12110)

* Add schedule to supported signing events

* Update cmd/image-builder/README.md

Co-authored-by: Iwona Langer <iwona.langer@sap.com>

---------

Co-authored-by: Wojciech Sołtys <74361703+Sawthis@users.noreply.github.com>
Co-authored-by: Iwona Langer <iwona.langer@sap.com>

cmd/image-builder/README.md

@@ -95,6 +95,7 @@ The Image Builder reusable workflow supports the following GitHub events to trig
 * `push` - to build images on push to the specified branch.
 * `pull_request_target` - to build images on pull requests.
 * `workflow_dispatch` - to manually trigger the workflow.
+* `schedule` - to build images on a regular basis.
 
 ## Reusable Workflow Reference
 
@@ -146,7 +147,7 @@ By default, Image Builder signs images with the production signify service.
 Image signing allows verification that the image comes from a trusted repository and has not been altered in the meantime.
 
 > [!NOTE]
-> Image Builder signs images built on the push and workflow_dispatch events only. Images built on the pull_request_target event are not signed.
+> Image Builder only signs images built on the `push`, `schedule`, and `workflow_dispatch` events. Images built on the `pull_request_target` event are not signed.
 
 ## Image Signing with Signify