Disable/move default webapps, per upstream recommendation

[tianon]

See https://tomcat.apache.org/tomcat-9.0-doc/security-howto.html#Default_web_applications

Closes #124

[tianon]

Note: if you are currently using any of these default webapps, this is a BREAKING CHANGE (intentionally).

They are still available within the image under webapps.dist and should be trivial to move/symlink/copy back into the webapps folder (which is why we're not removing them completely -- the space savings would only be ~7MB, which when compared with the hassle required to add these back, is considered worth it).

[tianon]

Thanks to docker-library/official-images#7217 this is now green. 👍

As an example of one easy way to restore some default example webapp (such as the manager applet):

FROM tomcat:9.0-jdk13-openjdk-oracle
RUN cp -avT $CATALINA_HOME/webapps.dist/manager $CATALINA_HOME/webapps/manager

A symlink would probably work as well, although that's untested.

[brontide]

For those looking to use the stock image and add manager back in you can do with with an altered entrypoint. Handy alternative to building a custom image just to have the manager.

services:
  tomcat:
    restart: unless-stopped
    image: tomcat:9
    volumes:
      - "./webapps:/usr/local/tomcat/webapps"
      - "./tomcat-users.xml:/usr/local/tomcat/conf/tomcat-users.xml"
    entrypoint: |
      bash -c 'cp -a webapps.dist/manager webapps/; exec catalina.sh run'