fix docker CI

apparently, github changed something in the runners, so that `maximize build disk space` is not able to free 35.84GB space on the root partition any more. Reduced the reserved space to 35GB to  fix this - hopefully leaving enough free space for building the docker image

.github/workflows/docker.yml

@@ -43,7 +43,7 @@ jobs:
           remove-android: true
           remove-haskell: true
           remove-codeql: true
-          root-reserve-mb: 35840
+          root-reserve-mb: 35000
           swap-size-mb: 1048
 
       - name: Check out the repo