Work around old apksigner on f-droid.org publish server

.gitlab-ci.yml

@@ -102,6 +102,27 @@ debian_testing:
     - ./run-tests
 
 
+# This tests with buster-backports to match what is used on the f-droid.org publish server. 
+buster_backports:
+  image: debian:buster-backports
+  <<: *apt-template
+  script:
+    - apt-get install
+        aapt
+        androguard
+        apksigner
+        dexdump
+        fdroidserver
+        git
+        gnupg
+        python3-defusedxml
+        python3-setuptools
+        zipalign
+    - apt-get install -t buster-backports apksigner
+    - cd tests
+    - ./run-tests
+
+
 # Test using latest LTS set up with the PPA, including Recommends.
 ubuntu_lts_ppa:
   image: ubuntu:latest