Transfer Azure batch windows container task output file to batch task working directory fails

I am prototyping an azure batch sample using windows container workload in batch task. This container expects an input file, executes an algorithm and writes an output file st01_demo_results.csv inside the container at path C:\app\output (this directory is created in dockerfile build). On batch task success, the output file will be uploaded to azure storage container using SAS URL. I have mostly used this sample to build my prototype, just that instead of using exe, I am using windows container. I am trying to transfer my output file generated inside windows container to batch task working directory by mounting a volume with source as batch task working directory and target my container output file path.

string taskCommandLine = $"{inputMediaFile} C:\\app\\output\\{outputMediaFile}";
string containerRunOptions = "-v %AZ_BATCH_TASK_WORKING_DIR%:c:\\app\\output:rw";

    var task = new CloudTask(taskId, taskCommandLine)
    {
      UserIdentity = new UserIdentity(autoUserSpec),
      ContainerSettings = containerSettings,
    };
    task.ResourceFiles = new List<ResourceFile> { inputFiles[i] };

With this, I get exception Unhandled exception. System.UnauthorizedAccessException: Access to the path 'c:\app\output\st01_demo_results.csv' is denied.

If, I don't provide volume binding in containerRunOptions then my container task executes fine but then I don't get the file transferred to my batch task working directory. I suspect, that azure batch somewhere ignores rw option and forcefully makes the volume binding read-only - because when I tried running the same windows container locally in docker desktop with ro option, I get same unauthorized exception.