better apt instrunctions for reduced image size

base/Dockerfile

@@ -3,17 +3,23 @@ FROM debian:9
 MAINTAINER Ivan Ermilov <[email protected]>
 MAINTAINER Giannis Mouchakis <[email protected]>
 
-RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends openjdk-8-jdk
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+      openjdk-8-jdk \
+      net-tools \
+      curl \
+      netcat \
+      gnupg \
+    && rm -rf /var/lib/apt/lists/*
+
 ENV JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64/
 
-RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends net-tools curl netcat gnupg
-
 RUN curl -O https://dist.apache.org/repos/dist/release/hadoop/common/KEYS
 
 RUN gpg --import KEYS
 
 ENV HADOOP_VERSION 3.1.1
 ENV HADOOP_URL https://www.apache.org/dist/hadoop/common/hadoop-$HADOOP_VERSION/hadoop-$HADOOP_VERSION.tar.gz
+
 RUN set -x \
     && curl -fSL "$HADOOP_URL" -o /tmp/hadoop.tar.gz \
     && curl -fSL "$HADOOP_URL.asc" -o /tmp/hadoop.tar.gz.asc \
@@ -22,18 +28,19 @@ RUN set -x \
     && rm /tmp/hadoop.tar.gz*
 
 RUN ln -s /opt/hadoop-$HADOOP_VERSION/etc/hadoop /etc/hadoop
+
 RUN mkdir /opt/hadoop-$HADOOP_VERSION/logs
 
 RUN mkdir /hadoop-data
 
 ENV HADOOP_PREFIX=/opt/hadoop-$HADOOP_VERSION
 ENV HADOOP_CONF_DIR=/etc/hadoop
 ENV MULTIHOMED_NETWORK=1
-
 ENV USER=root
 ENV PATH $HADOOP_PREFIX/bin/:$PATH
 
 ADD entrypoint.sh /entrypoint.sh
+
 RUN chmod a+x /entrypoint.sh
 
 ENTRYPOINT ["/entrypoint.sh"]