Add curl to ONOS docker image for use with healthchecks.

Change-Id: I78e482a76455ae40c0748a29d48e3e24c6de57e5
diff --git a/Dockerfile b/Dockerfile
index 47c1897..3e9b928 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -61,6 +61,9 @@
       org.label-schema.schema-version="1.0" \
       maintainer="onos-dev@onosproject.org"
 
+RUN apt-get update && apt-get install -y curl && \
+	rm -rf /var/lib/apt/lists/*
+
 # Install ONOS in /root/onos
 COPY --from=builder /output/ /root/onos/
 WORKDIR /root/onos