Update Dockerfile

Dockerfile

@@ -3,4 +3,4 @@ FROM golang:1.10.3
 ENV DEBIAN_FRONTEND=noninteractive 
 RUN 	go get -u github.com/golang/dep/cmd/dep  github.com/ugorji/go/codec/codecgen  \
 		&& echo deb http://deb.debian.org/debian stretch-backports main > /etc/apt/sources.list.d/backports.list \
-                && apt-get update && apt-get install -y libgs-dev ghostscript 
+                && apt-get update && apt-get install -y libgs-dev ghostscript wkhtmltopdf