The web.xml file of the engine-rest webapp is missing the XML declaration line (<?xml ... ?>) in the Tomcat Docker image:
The declaration line is there in the source of the file
https://github.com/camunda/camunda-bpm-platform/blob/master/engine-rest/assembly/src/main/webapp/WEB-INF/web.xml (is this the right file at all? it contains less stuff than the file in the image...).
So I guess it is lost when building the Docker image.