Use docker-ssh-agent-forward with Docker Compose

docker-compose.yml

version: '3'

services:
  app:
    image: gcea/ssh-client
    environment:
      SSH_AUTH_SOCK: /ssh-agent/ssh-agent.sock
    volumes:
      - ssh-agent:/ssh-agent

volumes:
  ssh-agent:
    external: true

example.sh

$ pinata-ssh-forward
ssh-agent
Agent forwarding successfully started.
Run "pinata-ssh-mount" to get a command-line fragment that
can be added to "docker run" to mount the SSH agent socket.

For example:
docker run -it `pinata-ssh-mount` ocaml/opam ssh git@github.com
$ docker-compose run --rm app ssh -o StrictHostKeyChecking=no git@github.com
Warning: Permanently added 'github.com,192.30.253.112' (RSA) to the list of known hosts.
PTY allocation request failed on channel 0
Hi djmaze! You've successfully authenticated, but GitHub does not provide shell access.
Connection to github.com closed.