Create some data to use in the build within the .ssh mounted directory.
docker run -it -v ~/.ssh:/vault/.ssh ubuntu /bin/bash -c "echo mysupersecret > /vault/.ssh/key"
Start a build secrets server. This server will be used by other containers are they're building built to access the .ssh mounted directory.
docker run -d -p 172.18.0.1:14242:3000 -v ~/.ssh:/vault/.ssh dockito/vault