Bugfix for dokku

See https://github.com/progrium/dokku/issues/49
Updated README.md
Download webapp-runner into app environment
Change maven command to package

clean install was taking too long and making it impossible
to cache node_modules.
Cache maven cache globally

This requires that the directory /global_cache is mounted into the build container.
If it is not, they are cached only in the project-specific cache directory.