ci: Run tests also on Debian 11

Debian 11 uses Python 3.9, which is not covered by any other CI images.

.github/compose/ci.docker-compose.yml

diff --git a/.github/compose/ci.docker-compose.yml b/.github/compose/ci.docker-compose.yml
index a295ecd..48d1d83 100644
--- a/.github/compose/ci.docker-compose.yml
+++ b/.github/compose/ci.docker-compose.yml
@@ -22,6 +22,10 @@
 
 services:
 
+  debian-11:
+    <<: *tests-template
+    image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-debian:11-${CI_IMAGE_VERSION:-latest}
+
   fedora-40:
     <<: *tests-template
     image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-fedora:40-${CI_IMAGE_VERSION:-latest}

.github/run-ci.sh

diff --git a/.github/run-ci.sh b/.github/run-ci.sh
index 07709de..3b3bb36 100755
--- a/.github/run-ci.sh
+++ b/.github/run-ci.sh
@@ -67,7 +67,7 @@
 
 
 if [ -z "${test_names}" ]; then
-    for test_name in "mypy fedora-40 fedora-missing-deps ubuntu-20.04"; do
+    for test_name in "mypy debian-11 fedora-40 fedora-missing-deps ubuntu-20.04"; do
 	if ! runTest "${test_name}"; then
 	    echo "Tests failed"
 	    exit 1

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5501137..c20bb57 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -36,6 +36,7 @@
         # The names here should map to a valid service defined in
         # "../compose/ci.docker-compose.yml"
         test-name:
+          - debian-11
           - fedora-40
           - fedora-missing-deps
           - ubuntu-20.04