CI: Allow a mirror to be specified for Docker Hub

To conserve bandwidth and potentially avoid rate limits, allow a local
mirror of Docker Hub to be specified globally. The default value is
unchanged.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

.gitlab-ci.yml

@@ -2,6 +2,7 @@
 
 variables:
   DEFAULT_TAG: ""
+  MIRROR_DOCKER: docker.io
 
 default:
   tags:
@@ -9,7 +10,7 @@ default:
 
 # Grab our configured image.  The source for this is found
 # in the u-boot tree at tools/docker/Dockerfile
-image: trini/u-boot-gitlab-ci-runner:jammy-20230308-21Mar2023
+image: ${MIRROR_DOCKER}/trini/u-boot-gitlab-ci-runner:jammy-20230308-21Mar2023
 
 # We run some tests in different order, to catch some failures quicker.
 stages: