RunTaskRequest on a awsx.ecs.FargateTaskDefinition with multiple containers ==> runs every command in every container on the task definition?

Are you asking whether you can specify which containers the command runs on? You can narrow down the command to task groups if you define those. See https://github.com/pulumi/pulumi-awsx/blob/333b1ff0d649e3f19187c2e8edc292dd6c131fbd/nodejs/awsx/ecs/taskDefinition.ts#L183