Update Dockerfile

Dockerfile

 FROM golang:1.11.4
 
 ENV DEBIAN_FRONTEND=noninteractive 
-
+RUN apt-get update && apt-get install -y libgs-dev ghostscript 
 RUN 	go get github.com/MrSaints/go-ghostscript/ghostscript \
         && go get -u github.com/golang/dep/cmd/dep \
 		&& echo deb http://deb.debian.org/debian stretch-backports main > /etc/apt/sources.list.d/backports.list \