CI: switch the debian stable image to i686

The debian image is used to test older dependencies and toolchains.
This adds an older architecture to that list.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index d78f8cd..5af6338 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -53,8 +53,8 @@
       - "_build/${CI_JOB_NAME}-report.xml"
       - "_coverage"
 
-debian-stable-x86_64:
-  image: registry.gitlab.gnome.org/gnome/glib/debian-stable:v1
+debian-stable-i686:
+  image: registry.gitlab.gnome.org/gnome/glib/debian-stable:v2
   stage: build
   except:
     - tags
diff --git a/.gitlab-ci/debian-stable.Dockerfile b/.gitlab-ci/debian-stable.Dockerfile
index f10ebc0..899d8cc 100644
--- a/.gitlab-ci/debian-stable.Dockerfile
+++ b/.gitlab-ci/debian-stable.Dockerfile
@@ -1,4 +1,4 @@
-FROM debian:stretch
+FROM i386/debian:stretch
 
 RUN apt-get update -qq && apt-get install --no-install-recommends -qq -y \
     bindfs \