http://manpages.ubuntu.com/manpages/natty/en/man8/apt-key.8.html:
OPTIONS
Note that options need to be defined before the commands described in
the previous section.
--keyring filename
With this option it is possible to specify a specific keyring file
the command should operate on. The default is that a command is
executed on the trusted.gpg file as well as on all parts in the
trusted.gpg.d directory, through trusted.gpg is the primary keyring