Currently the Dockerfile specifies the Tomcat version down to the patch level (TOMCAT_VERSION=8.0.20), which isn't ideal.
Tomcat 8.0.20 has by now several major security issues which are fixed in the lasted release 8.0.47.
I checked out the current github source and build / (shortly) tested successful with 8.0 and 8 as version tag. Which leads Docker to pull the current 8.0.47 respectively 8.5.23 tomcat image.
Considering the end of life for Apache Tomcat 8.0.x in June 2018 I would suggest changing the Tomcat version directly to the major 8 branch.
(furthermore an automated build up-to-date official guac image with an current tomcat would be good for everyone.)