Update to LTS Node version and provide API for setting node version

[bradfrosty]

The current version of Node in the docker image of this plugin is v12.18.3. Most importantly, this will exit the maintenance cycle on 2022/04/30 and thus stop receiving security updates.

Is there any mechanism (e.g. something like docker compose build config) to specify the installed Node version? This flexibility would be useful as developers may be running different versions, and things like npm lifecycle scripts may not work with Drone NPM node version.