Add an "alpha" image for Mono, representing the Mono Alpha apt repo

library/mono

@@ -22,3 +22,5 @@ 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