SSHKit::Runner::ExecuteError
Make sure that you have ssh-agent running on your machine, because you're using ssh forwarding, your local keys should be able to access and clone your repo from bitbucket. You can run agent it with:
eval `ssh-agent`ssh-add ~/.ssh/id_rsaMore info about ssh-forwarding you can find here.
Other thing you should check manually is that you can ssh to your server (your public key should be added to ~/.ssh/authorized_keys on server), and of course that you can clone your repo locally (that probably you're able to do).
Update:
Now i have realised that you turn forwarding for production, in that case you should make sure that key-pair that's on your server (you must have it, because forwarding is turned off), are able to clone repo from bitbucket, you can try cloning on server by hand too.
As the backtrace says, this is likely related to ssh public keys. I would recommend troubleshooting the problem with capistrano-ssh-doctor. You will get a report precisely saying what went wrong in the setup.
Disclaimer, I'm the author of capistrano-ssh-doctor plugin.