start using our stable-backports image (base-images#13)

.gitlab-ci.yml

@@ -57,7 +57,7 @@ linkcheck:
 # which generated it.
 mdlint:
   image:
-    name: debian:stable-backports
+    name: containers.torproject.org/tpo/tpa/base-images/debian:stable-backports
     entrypoint: [""]
   needs:
     - job: find-files
@@ -77,7 +77,7 @@ mdlint:
 # changed, so it doesn't require the above
 mdlintall:
   image:
-    name: debian:stable-backports
+    name: containers.torproject.org/tpo/tpa/base-images/debian:stable-backports
     entrypoint: [""]
   needs:
     - job: find-files