Removing cache from build, adding wheezy beta

Cached artifacts from the FSharp build weren't removed, wasting almost 200 MB.  Cleaning this up as well as environment variables that are used for the FSharp build only.

Also adding an image based on debian rather than larger ubuntu to beta test with users ahead of an official switch to debian based images.

library/fsharp

@@ -3,9 +3,13 @@ Maintainers: Dave Curylo <[email protected]> (@ninjarobot),
 GitRepo: https://github.com/fsprojects/docker-fsharp.git
 
 Tags: latest, 4, 4.1, 4.1.18
-GitCommit: a28196740e38035beea04c41d5862136413281e3
+GitCommit: 2201a2c453ca69249bf6f4c4c121e8bb2a847cf7
 Directory: 4.1.18/mono
 
+Tags: 4.1.18-wheezy-beta
+GitCommit: 0d86b281d7e8fccfe463d098310a5fd521f0276a
+Directory: 4.1.18/mono-wheezy
+
 Tags: 4.1.0.1
 GitCommit: a28196740e38035beea04c41d5862136413281e3
 Directory: 4.1.0.1/mono