Make the server's BASE_URL configurable

The "special" value of `ROOT` will serve requests on `/`.  Any other
value should serve on `/$BASE_URL` (as long as there are no slashes
in `BASE_URL` at least).

The default has been set to `plantuml` to better match documentation
in the README.md file.

Re plantuml#80, may help with plantuml#79.

Dockerfile.jetty

@@ -21,4 +21,5 @@ USER jetty
 
 ENV GRAPHVIZ_DOT=/usr/bin/dot
 
-COPY --from=builder /app/target/plantuml.war /var/lib/jetty/webapps/ROOT.war
+ARG BASE_URL=plantuml
+COPY --from=builder /app/target/plantuml.war /var/lib/jetty/webapps/$BASE_URL.war

Dockerfile.tomcat

@@ -17,5 +17,6 @@ RUN apt-get update && \
 
 ENV GRAPHVIZ_DOT=/usr/bin/dot
 
-RUN rm -rf /usr/local/tomcat/webapps/ROOT
-COPY --from=builder /app/target/plantuml.war /usr/local/tomcat/webapps/ROOT.war
+ARG BASE_URL=plantuml
+RUN rm -rf /usr/local/tomcat/webapps/$BASE_URL
+COPY --from=builder /app/target/plantuml.war /usr/local/tomcat/webapps/$BASE_URL.war