Run diffoscope as non root user in container by using USER instruction

Diffoscope running by default in container (from the command in diffoscope.org website)

docker run --rm -t -w $(pwd) -v $(pwd):$(pwd):ro \ registry.salsa.debian.org/reproducible-builds/diffoscope file1 file2

will run the diffoscope inside the container as root, which is bad as diffoscope doesn't require root. to fix it, add the USER instruction to Dockerfile, see: https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#user https://docs.docker.com/engine/reference/builder/#user