Do not copy any git ref in Docker images

Theory here is that this is what makes caching fail, because
HEAD changes at every push

.dockerignore

@@ -19,13 +19,8 @@
 .DS_Store
 **/.DS_Store
 
-# Ignore git, but keep git HEAD and refs to access current commit hash if needed:
+# Ignore git
-#
-# $ cat .git/HEAD | awk '{print ".git/"$2}' | xargs cat
-# d0b8727759e1e0e7aa3d41707d12376e373d5ecc
 .git
-!.git/HEAD
-!.git/refs
 
 # Common development/test artifacts
 /cover/