ci: Fix debian32 job

Gitlab runner fails to pull the image due to:
no matching manifest for linux/amd64 in the
manifest list entries. Set the architecture of the
image to pull, see:
https://docs.gitlab.com/ee/ci/yaml/#imagedocker

Signed-off-by: Michael Scherle <michael.scherle@rz.uni-freiburg.de>

.gitlab-ci.yml

@@ -136,7 +136,10 @@ makecheck-debian32:
       python3-six python3-pyparsing liborc-0.4-dev meson git
       g++ autoconf-archive
     - *protocol
-  image: i386/debian:latest
+  image:
+    name: i386/debian:latest
+    docker:
+      platform: i386
   script:
   - git clean -fdx
   - git submodule foreach --recursive git clean -fdx