Previously, the help information for buildx indicated that users could
create a new instance of the docker driver - which is explicitly
something we don't support, driver of this form are automatically
derived from the available list of docker contexts.
This patch ensures that don't have AllowsInstance set will not appear in
the help text, and additionally provide a new more specific error
message instead of the generic "failed to find driver". This should help
point users in the correct direction.
Signed-off-by: Justin Chadwell <me@jedevc.com>
17d4369866