Install node.js and npm into build environment, run npm install
Download webapp-runner into app environment
Bugfix for dokku

See https://github.com/progrium/dokku/issues/49
Cache node_modules and bower cache
Change maven command to package

clean install was taking too long and making it impossible
to cache node_modules.
Cache npm, bower and maven caches 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.
Allow specifying subdirectory for node.js stuff

This means that in the .env file a directory can be defined like
 export NPM_DIR=backend
and then package.json, .npmrc and node_modules directories can be
placed in that directory (in this example, "backend" directory).
Add a tip about the package.json in subdirectory feature
Only install production dependencies with npm
Cache bower only locally

Caching bower cache directory seemed to result in some
race conditions (possibly when multiple instances of it
are run at the same time).
This manifested as missing files from the cache directory.
Temporarily remove bower caching

At least until issues in bower are resolved.
See the following issues on bower's github:
933, 991
Install a fork of webapp-runner

This one allows setting uriEncoding on the command line:
--uri-encoding UTF-8