add unprivilegeduser via the Dockerfile

This changes the test TestBuildWithInaccessibleFilesInContext to not
add the user 'unprivilegeduser' and add it via the Dockerfile instead.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5914 from unclejack/fix_unprivileged_user_creation

add unprivilegeduser via the Dockerfile