Improve documentation on referencing build artifacts

[boskoop]

See:

• Artifact not copied when using Maven ${project.build.directory} property #101
• Using mave in the buildArgs parameter of configuration, can't get ${project.basedir} normally #209

[mattnworb]

Thanks for adding this, and thanks also for including the link to #101 (I had forgotten about that issue which recaps the story better than in #209)