update Dockerfile to use latest Tomcat 9.0 (no version bump)
diff --git a/Dockerfile b/Dockerfile
index 06555da..9a7e40b 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -25,7 +25,7 @@
 # fastest, minimum build
   && mvn clean package -pl jspwiki-war,jspwiki-wikipages/en -am -DskipTests
 
-FROM tomcat:8.5
+FROM tomcat:9.0
 
 COPY --from=package /tmp/jspwiki-war/target/JSPWiki.war /tmp
 COPY --from=package /tmp/jspwiki-wikipages/en/target/jspwiki-wikipages-en-*.zip /tmp