[Feature Request] SSH Private Key in Environment Variable

[bkuhl]

It's possible to pull a private key from an environment variable and set it when the container runs. An example of this is at https://github.com/realpage/asset-publisher/blob/master/infrastructure/start.sh#L14. The Dockerfile's CMD is then set to run the script before hitting the process that Docker keeps running. Implementing this step would be a great step towards this container not having to be rebuilt with custom config.