add grsec compatibility to crashplan container

[Klowner]

Thanks so much for your containers, I've found them to be super useful.

I run Docker on Alpine with a hardened kernel, and since Java does some weird memory management stuff it requires disabling MPROTECT for grsec hosts. It's a small change, but it may broaden your audience :+1:

Thanks again!

[gfjardim]

Done it by adding a variable. Please read the README for instructions.

Best regards!

[Klowner]

Awesome, thanks!