Reduce Docker layers and optimize apt usage

Dockerfile

@@ -1,12 +1,14 @@
 FROM ubuntu:16.04
 MAINTAINER Barna Csorogi <barnacs@justletit.be>
 
-RUN DEBIAN_FRONTEND=noninteractive apt-get update
-RUN DEBIAN_FRONTEND=noninteractive apt-get upgrade -y
-RUN DEBIAN_FRONTEND=noninteractive apt-get install -y \
-    libjpeg8 \
-    openssl \
-    ssl-cert
+RUN DEBIAN_FRONTEND=noninteractive apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get upgrade -y && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y \
+        libjpeg8 \
+        openssl \
+        ssl-cert && \
+    DEBIAN_FRONTEND=noninteractive apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
 
 WORKDIR /opt/compy
 COPY \