I implemented a solution to this problem using docker-ssh-agent-forward for using ssh-agent at runtime and committing intermediary images for builds (yes, docker build). This should be more robust than using socat for concurrent builds (socat can only handle single connections unless you are using fork which complicates things further) and more secure. It works great on Docker for Mac and Linux alike.
I have posted complete solution with documentation, examples and base Dockerfile for node/npm here: https://github.com/iheartradio/docker-node
It can of course be extended to other development environments.