Email picked up by Postfix locally goes through transport "pickup" (defined in /etc/postfix/master.cf). If you want to sign it, modify /etc/postfix/master.cf like this:
*) Find the "pickup" transport, like below:
pickup unix n - y 60 1 pickup
*) Append one line RIGHT AFTER it:
pickup unix n - y 60 1 pickup
-o content_filter=smtp-amavis:[127.0.0.1]:10026
Warning: there must be at least one whitespace before '-o'.
*) Restart Postfix service.
With this change, ALL emails sent with 'sendmail' or 'mail' programs (or PHP mail function, etc) will go through Amavisd (sign DKIM signature, and more).