Fix rendering of non-sequence diagrams

Although sequence diagrams render fine without, any(?) other diagram
type requires the  `GRAPHVIZ_DOT` environment variable to be set for
the server to produce usable output.

Dockerfile.jetty

@@ -19,4 +19,6 @@ RUN apt-get update && \
 
 USER jetty
 
+ENV GRAPHVIZ_DOT=/usr/bin/dot
+
 COPY --from=builder /app/target/plantuml.war /var/lib/jetty/webapps/ROOT.war