Add a docker job property, so we can set docker configuration on a
per-job basis.

Signed-off-by: Nigel Magnay <nigel.magnay@gmail.com>
Revise the post-completion options (tag, push, clean) - no longer
part of the template, but part of the job.

Signed-off-by: Nigel Magnay <nigel.magnay@gmail.com>