Discourage use of the onbuild image for production use

[Starefossen]

While the onbuild variant is really useful for "getting off the ground running" (zero to Dockerized in a short period of time), it's not recommended for long-term usage within a project due to the lack of control over when the ONBUILD triggers fire (see also moby/moby#5714, moby/moby#8240, moby/moby#11917).

We should backport the enhanced description of the onbuild image variant which used in the documentation of the Node.js Docker Image in the Docker Registry to this repository: https://github.com/docker-library/docs/tree/master/node#nodeonbuild

[Starefossen]

Closing since #80 has been merged.