Another thing you can do in ssh-find-agent.sh is create a symbolic link to ${SSH_AUTH_SOCK} – instead of setting and reading in the path, you can assume it always exists in /tmp/agent.sock.
ln -sf $SSH_AUTH_SOCK /tmp/agent.sock
then…
docker run -it \
-v ${LOCAL_STATE}:/tmp \
-e SSH_AUTH_SOCK=/tmp/agent.sock \
golang ssh -T git@github.com
This has also made defining the path in compose files quite trivial…