0e06c1cad1
The daemon uses two similar filename extensions to identify different
kinds of certificates. ".crt" files are interpreted as CA certificates,
and ".cert" files are interprted as client certificates. If a CA
certificate is accidentally given the extension ".cert", it will lead to
the following error message:
Missing key ca.key for certificate ca.cert
To make this slightly less confusing, clarify the error message with a
note that CA certificates should use the extension ".crt".
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>