[AIRFLOW-6818] Fix for old versions of git on Dockerhub builds (#7440)

Dockerhub uses an old version of git (2.7.4 as of our testing in Feb
2020) so needs a slightly different syntax for git ls-files with an
exclusion. This works with current-latest git (2.25.0) too

hooks/build

@@ -397,7 +397,7 @@ function filterout_deleted_files {
 }
 
 # This deals with files
-git ls-files -z ':!:airflow/www/static/docs' | filterout_deleted_files | xargs -t -0 chmod og-w
+git ls-files -z -- ./ ':!:airflow/www/static/docs' | filterout_deleted_files | xargs -0 chmod og-w
 # and this deals with directories
 git ls-tree -z -r -d --name-only HEAD | filterout_deleted_files | xargs -t -0 chmod og-w,og+x