GUACAMOLE-1005: Docker, configure RemoteIPValve with xmlstarlet

Dockerfile

@@ -50,6 +50,9 @@ RUN /opt/guacamole/bin/build-guacamole.sh "$BUILD_DIR" /opt/guacamole "$BUILD_PR
 # For the runtime image, we start with the official Tomcat distribution
 FROM tomcat:${TOMCAT_VERSION}-${TOMCAT_JRE}
 
+# Install XMLStarlet for server.conf alterations
+RUN apt-get update -qq && apt-get install -y xmlstarlet
+
 # This is where the build artifacts go in the runtime image
 WORKDIR /opt/guacamole
 
@@ -68,4 +71,3 @@ USER guacamole
 # Start Guacamole under Tomcat, listening on 0.0.0.0:8080
 EXPOSE 8080
 CMD ["/opt/guacamole/bin/start.sh" ]
-