A stack is a directory of compose files; the current directory is a stack when
it holds one. The same command works for a project, a catalogue stack, a group
and a host singleton, and stacks sharing a project are now a single compose
call.
Two traps of the make engine are closed on the way: an unknown command is an
error instead of a silent success, and a group is only expanded when its name
is lowercase, so an environment variable can no longer be mistaken for one.
That guard spells out its character class because a-z matches uppercase too
under a dictionary collation.
