Working around Docker's COPY command copying as root using GNU M4

I’ve been working with Docker a lot recently, and have been switching to non-root users within the Dockerfile using the USER instruction (not for security reasons, but in order to test software not as root). As a result, I’ve been suffering a little from this issue - essentially, files copied into a Docker image using ADD or COPY are copied with the owner as root, irrespective of the ownership outside the Docker image.