Install CA certs, curl and ping in Dgraph Docker image.

contrib/Dockerfile

@@ -7,6 +7,10 @@
 FROM ubuntu:latest
 MAINTAINER Dgraph Labs <contact@dgraph.io>
 
+RUN apt-get update && \
+  apt-get install -y --no-install-recommends ca-certificates curl iputils-ping && \
+  rm -rf /var/lib/apt/lists/*
+
 ADD linux /usr/local/bin
 
 EXPOSE 8080