Fix Docker cleanup to preserve master tag

Author: Tschuppi81

.github/workflows/build-and-push.yml

@@ -213,5 +213,5 @@ jobs:
           package-name: 'onegov-cloud'
           package-type: 'container'
           min-versions-to-keep: 20
-          ignore-versions: '^release-.*$'
+          ignore-versions: '^(release-.*|master)$'
           ignore-versions-include-tags: true