Add building documentation

[lemeurherve]

Describe your use-case which is not covered by existing documentation.

There is currently no documentation how to build images and develop in this repository.

It would be nice to have a document like https://github.com/jenkinsci/docker/blob/master/HACKING.adoc or a section like https://github.com/jenkinsci/docker-ssh-agent#building-instructions in the main README.md.

Are you interested in contributing to the documentation?

Yes:

• chore: align with docker-ssh-agent build process #827

[lemeurherveCB]

Fixed with #827, can be closed.