Tests currently rely on a local Tomcat installation.
Of the other alternatives:
Docker has a problem with the COPY instruction causing an aufs error - this is intermittent and has been observed and here by others
The gradle-tomcat plugin starts and stops an embedded Tomcat instance, but the code just hangs, even though it is the exact same code and Tomcat version as the local install
The code will not run on Payara (the embedded server used by the Vaadin-Gradle plugin) - a checked exception in Shiro prevents it
Tests currently rely on a local Tomcat installation.
Of the other alternatives: