add a dockerignore

This is strictly for tightening the container to be more hygenic.

Signed-off-by: Brian Harring <ferringb@gmail.com>

.dockerignore

@@ -0,0 +1,11 @@
+# Actual layer caching is impossible due to .git, but
+# that must be included for provenance reasons.  These ignores
+# are strictly for hygenic build.
+*
+!/*.go
+!/go.*
+!/cmd/*
+!/internal/*
+!/helpers/*
+!/VERSION
+!/.git/