Update Dockerfile to Ubuntu 18.04 and cleanup syntax

Author: djds

docker/Dockerfile

@@ -1,4 +1,6 @@
-FROM ubuntu:14.04
+FROM ubuntu:18.04
+
+ENV DEBIAN_FRONTEND="noninteractive"
 
 # install depdencies
 RUN apt-get update && apt-get install -y \
@@ -12,17 +14,17 @@ RUN apt-get update && apt-get install -y \
   git
 
 # Download the latest version of the code from GitHub
-WORKDIR /opt/
-RUN git clone https://github.com/USArmyResearchLab/Dshell.git
+RUN git -C /opt clone https://github.com/USArmyResearchLab/Dshell.git
 
 # download and untar GeoIP files
 WORKDIR /opt/Dshell/share/GeoIP/
-RUN wget https://geolite.maxmind.com/download/geoip/database/GeoLite2-Country.tar.gz
-RUN wget https://geolite.maxmind.com/download/geoip/database/GeoLite2-ASN.tar.gz
-RUN tar -zxf GeoLite2-Country.tar.gz
-RUN tar -zxf GeoLite2-ASN.tar.gz
-RUN GeoLite2-Country*/GeoLite2-Country.mmdb .
-RUN GeoLite2-ASN*/GeoLite2-ASN.mmdb .
+RUN wget https://geolite.maxmind.com/download/geoip/database/GeoLite2-Country.tar.gz \
+    && wget https://geolite.maxmind.com/download/geoip/database/GeoLite2-ASN.tar.gz \
+    && tar -zxf GeoLite2-Country.tar.gz \
+    && tar -zxf GeoLite2-ASN.tar.gz \
+    && ln -s GeoLite2-Country*/GeoLite2-Country.mmdb . \
+    && ln -s GeoLite2-ASN*/GeoLite2-ASN.mmdb . \
+    && rm -rf /var/lib/apt/lists/*
 
 # make Dshell
 WORKDIR /opt/Dshell/
@@ -31,4 +33,4 @@ RUN make
 # Used to mount pcap from a host OS directory
 VOLUME ["/mnt/pcap"]
 
-CMD ["/opt/Dshell/dshell"]
+ENTRYPOINT ["/opt/Dshell/dshell"]