utils/docker-run: allow IMAGE to be passed in the environment

It is sometimes useful to use docker-run, but with a different image
than the default one. This commit allows to override the image being
used by only defining IMAGE if not already passed in the environment.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr: s/\t/    /g]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

utils/docker-run

@@ -9,9 +9,11 @@ else
     # Support git-worktree
     GIT_DIR="$(cd "${MAIN_DIR}" && git rev-parse --no-flags --git-common-dir)"
 fi
-# shellcheck disable=SC2016
-IMAGE=$(grep ^image: "${MAIN_DIR}/.gitlab-ci.yml" | \
-        sed -e 's,^image: ,,g' | sed -e 's,\$CI_REGISTRY,registry.gitlab.com,g')
+if test -z "${IMAGE}" ; then
+    # shellcheck disable=SC2016
+    IMAGE=$(grep ^image: "${MAIN_DIR}/.gitlab-ci.yml" | \
+            sed -e 's,^image: ,,g' | sed -e 's,\$CI_REGISTRY,registry.gitlab.com,g')
+fi
 
 declare -a docker_opts=(
     -i