fix(yocto): make SSH_AUTH_SOCK available within Docker

When the SSH private key is encrypted by a passphrase, OpenSSH would ask for that passphrase when repo is updating the Yocto sources. This currently fails, probably because repo within the Docker container within fwfwk does not provide an interactive shell.

This can be avoided when an active SSH agent has unlocked the key. Make SSH_AUTH_SOCK available within the Docker container, so that the unlocked key can be used.