docker: silence a warning when fixing ownership

Silence this warning:

> chown -R 1000.1000 /Unvanquished/build/release
> chown: warning: '.' should be ':': '1000.1000'

Author: illwieckz

docker/fix-ownership

@@ -28,6 +28,6 @@ source "$(dirname "${BASH_SOURCE[0]}")/common.sh"
 
 if [ -d '/Unvanquished/build/release' ]
 then
-	ownership="$(stat --format='%g.%u' '/Unvanquished/build/release')"
+	ownership="$(stat --format='%g:%u' '/Unvanquished/build/release')"
 	_exec chown -R "${ownership}" '/Unvanquished/build/release'
 fi