As the last layer created by
docker build -t mytag .
---> Running in d9a42e53eb5a
You just launch the last created image with a shell and launch the command, and you will have a more clear error message
docker run -it d9a42e53eb5a /bin/bash
(this assumes /bin/bash is available, it may be /bin/sh or anything else)
and with the prompt, you launch the last failing command, and see what is displayed