Add xvfb to build environment

concourse/docker/Dockerfile

@@ -8,7 +8,9 @@ RUN apt-get update && apt-get install -y \
   git \
   jq \
   openjdk-8-jdk \
-  curl
+  curl \
+  xvfb \
+  icewm icewm-themes
 
 RUN curl -sL https://deb.nodesource.com/setup_8.x | bash - \
   && apt-get install -y nodejs