Merge pull request #3573 from octo/ci/buster

.cirrus.yml: Add Debian Buster.

.cirrus.yml

@@ -9,7 +9,7 @@ env:
 #
 release_ready_task:
   container:
-    image: collectd/ci:stretch_amd64
+    image: collectd/ci:buster_amd64
   lint_script:
     - /checks/check-bashisms.sh
     - /checks/check-pod.sh
@@ -26,18 +26,21 @@ release_ready_task:
 #
 debian_default_toolchain_task:
   matrix:
-    - allow_failures: false
-      container:
+    - container:
+        image: collectd/ci:buster_amd64
+      allow_failures: false
+    - container:
         image: collectd/ci:stretch_amd64
-    - allow_failures: false
-      container:
+      allow_failures: false
+    - container:
         image: collectd/ci:stretch_i386
-    - allow_failures: false
-      container:
+      allow_failures: false
+    - container:
         image: collectd/ci:trusty_amd64
-    - allow_failures: false
-      container:
+      allow_failures: false
+    - container:
         image: collectd/ci:xenial_amd64
+      allow_failures: false
     # debian/unstable is expected to fail
     - allow_failures: true
       skip_notifications: true