.github/workflows/docker.yml: Adjust easimon/maximize-build-space root-reserve-mb for new GitHub runners

Author: Matthias Koeppe

.github/workflows/docker.yml

@@ -155,7 +155,7 @@ jobs:
         uses: easimon/maximize-build-space@v10
         with:
           # need space in /var for Docker images
-          root-reserve-mb:      40000
+          root-reserve-mb:      30000
           remove-dotnet:        true
           remove-android:       true
           remove-haskell:       true