Clean up image, reduce size

SteamCMD-Installer/Dockerfile

@@ -20,4 +20,12 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         lib32gcc-s1 \
         steamcmd && \
     ln -s /usr/games/steamcmd /bin/steamcmd && \
-    useradd ${USER} -d ${HOME}
+    useradd -U -d ${HOME} -s /bin/false ${USER} && \
+    # Clean up image
+    apt-get autoremove && \
+    apt-get clean && \
+    rm -rf \
+        /tmp/* \
+        /var/lib/apt/lists/* \
+        /var/tmp/* \
+        /var/log/*