Build: Set a fixed umask before starting Docker

Umask seems to leak into Docker containers and causes the build to
not be reproducible accross different umasks.

-----

taken from Electron-Cash/Electron-Cash@984967b408

contrib/build_tools_util.sh

@@ -1,5 +1,8 @@
 #!/usr/bin/env bash
 
+# Set a fixed umask as this leaks into docker containers
+umask 0022
+
 RED='\033[0;31m'
 BLUE='\033[0;34m'
 YELLOW='\033[0;33m'