Docker build of guacamole-client broken by recent update to maven:3-jdk-8

The guacamole-client Docker build uses the maven:3-jdk-8 image as the build image. That image is currently broken for any Maven build involving tests due to SUREFIRE-1588, a bug in the Maven surefire plugin which results in a hard failure due to a bug in the updated version of OpenJDK 8 present on this image.

This will presumably also break the guacamole-client build on any system with that version of OpenJDK 8.