docker-credential-helper-ecr: declare conflict with cask docker

Author: khipp

Formula/d/docker-credential-helper-ecr.rb

@@ -23,6 +23,8 @@ class DockerCredentialHelperEcr < Formula
 
   depends_on "go" => :build
 
+  conflicts_with cask: "docker"
+
   def install
     (buildpath/"GITCOMMIT_SHA").write tap.user
     system "make", "build"