Add some docs about which env vars are defined in new containers

@SvenDowideit FYI

Closes #3087

Signed-off-by: Doug Davis <dug@us.ibm.com>
Actually, I'd prefer to keep the system defined ones first.  I think it makes more sense to have the ones that will _always_ appear first, as they will always apply in all cases, then the optional ones second.  But if you feel strongly about I, as the newbie :-), will switch it.