Helper image container should always use native platform
What does this MR do?
Ensures the helper image always uses the native platform.
Why was this MR needed?
We pull the image with the native platform (unless imported). We need to ensure we always run it with the native platform too.
Service and main job containers can continue to optionally specify the platform.
This is needed because platform needs to always be optional as it is unsupported by older docker daemons.