Permission denied (publickey) error when doing cap deploy

I encountered this when trying to do a capistrano deploy on one of client's server. This error means that the server that's trying to fetch from repository on Github does not have permission to do so. We can resolve this with either:

• Adding server's public key to the Github repo's list of deploy keys.
• Use ssh agent forwarding so Github thinks it's you who's doing the deploy.

If ssh-agent isn't running, run the following in terminal.

exec ssh-agent bash