changes to the Readme for custom agent containers?

[caljnj]

nothing in the Readme (nor in bootstrap-via-installer or bootstrap-with-installed-agent) helped with custom agent containers to work with the elastic agent.

after spending a whole day knocking my head against this, all it took was to:

• build my own base image
• use my own image as the source in the debian/ubuntu agent Dockerfiles e.g. https://github.com/gocd/docker-gocd-agent-ubuntu-18.04

is the stuff in the readme still useful? should i propose a change?