utils/docker-run: fix shellcheck errors

In utils/docker-run line 10:
    --user $(id -u):$(id -g) \
           ^------^ SC2046: Quote this to prevent word splitting.
                    ^------^ SC2046: Quote this to prevent word splitting.

The suggestions from shellcheck can be applied.

Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

utils/docker-run

@@ -7,7 +7,7 @@ IMAGE=$(grep ^image: "${MAIN_DIR}/.gitlab-ci.yml" | \
         sed -e 's,^image: ,,g' | sed -e 's,\$CI_REGISTRY,registry.gitlab.com,g')
 
 exec docker run -it --rm \
-    --user $(id -u):$(id -g) \
+    --user "$(id -u):$(id -g)" \
     --mount "type=bind,src=${MAIN_DIR},dst=${MAIN_DIR}" \
     --workdir "${MAIN_DIR}" \
     "${IMAGE}" "${@}"