Adding a platform-dependant executable to a Jib image

We have a multiplatform Jib build (linux/arm64 and linux/amd64), to which we need to add an executable (the AWS ecr-credentials-helper for Docker, to be precise). However, there does not seem to be a way to do this; Adding files can only be done generically.

What I've considered:
- Use `extraDirectories`: this only allows selecting files and setting their permissions.
- Write my own extension: the `ContainerBuildPlan` you can modify contains both platforms. There does not seem to be a way to modify the container build plan of individual containers - only the generic plan.
- Create a custom base image which already includes this executable: still considering, but this would involve another step when we want to update the underlying JRE image by updating and publishing our own new base image.