Minor docker changes.

Dockerfile

 ARG R_TAG=3.5.2
-FROM rocker/shiny:$R_TAG AS builder
+FROM rocker/shiny:$R_TAG
 
 # Install and build R packages from CRAN and Bioconductor, removing documentation and tests
 RUN R -e 'install.packages( \
@@ -32,12 +32,13 @@ ADD demo /tmp/demo
 RUN cd /tmp/demo \
     && R -f make-demo.R \
     && mkdir -p /srv/shiny-server/demo \
-    && ls -la && cp demo.RData /srv/shiny-server/demo
+    && ls -la && cp demo.RData /srv/shiny-server/demo \
+    && rm -rf /tmp/demo
 
 # Fix file permissions
 RUN chmod -R a+rX /srv/shiny-server/ \
     && chgrp shiny /var/lib/shiny-server \
     && chmod g+rwxs /var/lib/shiny-server
 
-# Run with a non-privileged user
+# Run with a unprivileged user
 USER shiny