Adding info to the "Extending Jenkinsfile Runner Docker images" document

[krisstern]

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

As stated in the documentation on "Extending Jenkinsfile Runner Docker images"'s "Adding Tools" section, the sections is incomplete and requires filling in with the appropriate details. This issue would be ideal for any newbies to the project as an introduction.

Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

The entire "Extending Docker" document except the "Adding Tools" section should be studied carefully -- including the links contained there -- for the context, and could serve as a starting point for the details to be supplied to complete the section.