Pin a specific version on the apt-get install

library/mono

@@ -23,4 +23,4 @@ onbuild: git://github.com/mono/docker@adc7a3ec47f7d590f75a4dec0203a2103daf8db0 3
 3.8.0-onbuild: git://github.com/mono/docker@adc7a3ec47f7d590f75a4dec0203a2103daf8db0 3.8.0/onbuild
 3.8-onbuild: git://github.com/mono/docker@adc7a3ec47f7d590f75a4dec0203a2103daf8db0 3.8.0/onbuild
 
-alpha: git://github.com/mono/docker@c22b84ebdbc30466425d8483de24054d4601b76f alpha
+alpha: git://github.com/mono/docker@f639c9ba71c3e3601c079b353e12351bd008b11e alpha