Fix sanity file generation in Dockerfile (#610)

We no longer ship the sanify file as part of the SDK but
expect it be generated on first use:
  https://github.com/WebAssembly/waterfall/pull/656

Also remove the Dockerfile itself from the docker image if
we include it then whenever you change anything in the
Dockerfile it invalides all the layers and forces a
completely rebuild.  This makes iterating on the image
very slow.

.dockerignore

@@ -4,9 +4,6 @@
 # Allow to run the test script inside the Docker container
 !/docker/test_dockerimage.sh
 
-# Allow the Dockerfile for future re-creation/reference
-!/docker/Dockerfile
-
 # Ignore unnecessary files inside top-level directory
 *.bat
 *.csh