Allow use of ssh-agent instead of mounting SSH key
Mounting an SSH key into the docker-ssh-tunnel container requires that it be unprotected by a passphrase, or for the passphrase to be stored in the container config. This is insecure.
As an alternative, this change allows the user to forward their ssh key agent SSH_AUTH_SOCK into the container. When this environment variable is defined, the ssh command in run.sh will not attempt to use a private key file, but instead will use the default behaviour of looking for an agent socket at the path ${SSH_AUTH_SOCK}.
Successfully tested on Docker Desktop for Mac.
I've cleaned up the PR to make the change as minimal as possible while providing useful command options to the README.md.
I've updated the example in the docs to use the user's ${SSH_AUTH_SOCK} env var instead of hard-coding the path.
This works in WSL (Ubuntu:22.04 with docker integration enabled); as well as on a Mac, so it's more cross-platform now.