[docker-prepare] Use runuser instead of sudo

sudo may not be available in the image, and runuser is preferred for
this use-case anyway.

tools/docker-prepare

@@ -9,5 +9,12 @@ shift
 
 cd "${OUTSIDE_EMPER_ROOT}"
 
-# shellcheck disable=SC2068
-exec sudo -u user $@
+if [[ $@ ]]; then
+	exec runuser -u user -- $@
+else
+	# use --login so that clang is in path
+	runuser -u user -- /bin/bash --rcfile <(echo "cd ${OUTSIDE_EMPER_ROOT}") -l
+fi
+
+# Fallback into a root shell
+/bin/bash -l