One of the first stumbling blocks, when using Docker, is often discovering that your SSH keys are on the wrong side of the hatchway.

A simple solution is just to copy them, when building the image:

COPY ~/.ssh /root/.ssh

And this is reasonable, if the image is never going to leave your laptop. Any respectable tinfoil hat wearing neckbeard will look for an alternative though.

An obvious next step is to mount the same folder, as a volume:

SSH=~/.ssh
docker run -it --rm -v ${SSH}:/root/.ssh app npm install

Unfortunately, you’ll probably find at this point that the permissions are not correct inside the container, and that if you try and change them, they are hopelessly entangled with the permissions outside the container.

After much googling, and trying ideas from StackOverflow, the best solution I found was to use the same ssh-agent as the host:

SSH=~/.ssh
docker run -it --rm -v ${SSH}:/root/.ssh -v $SSH_AUTH_SOCK:/ssh-agent -e SSH_AUTH_SOCK=/ssh-agent app npm install

(I’m still mounting the .ssh folder, but only to recycle my known_hosts file).

Or, using compose:

version: '2' 
services:
  app:
    build: .
    volumes:
      - .:/app
      - ~/.ssh:/root/.ssh
      - $SSH_AUTH_SOCK:/ssh-agent