umask as env variable

[Gjermundhp02]

Enhancement Type

Improve an existing feature

Describe the enhancement

For a project I'm doing i need more restrictive file permissions in the data directory. (750)

umask is already in the /start file and it would be nice to have it as an environment variable.

-umask 0002
+umask "${UMASK:=0002}"

[itzg]

Great idea. A PR would be glad accepted if you want to submit that.