Docker-worker currently runs tasks as root, but we don't want that for desktop tests, at least. There's a workaround in place in testing/docker/desktop-build/bin/build.sh which switches users before doing anything else. We could easily do something similar for tests. The question is, do we want to make this change for all users of the 'tester' image, or make it optional, or use a different image for desktop and B2G.
I want to do bug 1213325 first, as that will bring the test scripts in line with the build scripts and make this feature easier.
Actually, bug 1213325 accomplished this.