Dockerfile: upgrade esy to 0.6.4 (#58)

Author: cknitt

docker/Dockerfile

@@ -4,4 +4,4 @@ RUN apt-get update \
   && DEBIAN_FRONTEND=noninteractive apt-get install -y nodejs npm git rsync m4 curl zip \
   && rm -rf /var/lib/apt/lists/*
 
-RUN npm i -g esy@0.6.2
+RUN npm i -g esy@0.6.4