Files can now have a relative path in the tar that gets sent to the buil...

...d agent. This solves the case where a Dockerfile contains "ONBUILD ADD target/x /y". Before, the build could not find "target/x" because the file in the tar did not contain a relative path. This way, both "docker build ." and "mvn package" work with the same Dockerfile.

- DockerFile as a separate configuration attribute
- Artifacts ("files") can now have a destination in the tar

Better error message when one of the files is not present