Change file's owner when copying resources with testcontainers

Question

I'm using testcontainers in Java tests. To configure application in container I need to put and mount configuration files: some files are static, so I'm mounting them using withClassPathResourceMapping when creating new container:

container.withClassPathResourceMapping(
  "/path/to/res",
  "/etc/app/config.name",
  BindMode.READ_ONLY
)

Other files are generated dynamically and could be overridden by app in container, so I'm copying the content to container using copyFileToContainer after container start:

container.copyFileToContainer(
 Transferable.of(bin /* byte[] */),
 "/var/app/resource.name"
)

The app is running in container as app:app user and group which are defined in the Dockerfile.

And I have two similar problems here:

1. withClassPathResourceMapping operation creates missed directory if not found, e.g. classpath mapping for "/etc/app/config.name" creates "/etc/app/ directory. But it creates these directories as root:root user, so app can't create new files in this directory later
2. the files which are copied into container using copyFileToContainer are not read-only and can be modified by app. But copyFileToContainer creates files as root:root user, so app can't write to these files.

I tried to exec chown -R /path after container start but this command fails, since the exec user is not a root.

What is the correct way to setting owner and permissions in testcontainers?