ci: trigger build-multi-stage on ready_for_review event

Ensure the workflow runs when a PR transitions from draft to ready state
by adding the ready_for_review event type to the pull_request trigger.

Assisted-by: AskBob <askbob@ibm.com>
Signed-off-by: Niels de Vos <ndevos@ibm.com>

Author: nixpanic

.github/workflows/build-multi-stage.yaml

@@ -3,6 +3,7 @@ name: multi-arch-build
 # yamllint disable-line rule:truthy
 on:
   pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
     branches:
       - '*'
 permissions: