Using an ssh-agent container in conjunction with docker-compose

docker-compose.yml

# This compose file mounts an SSH agent socket from the named volume "ssh" into a Ruby container.
# The SSH_AUTH_SOCK variable is set so everything just works.
app:
  image: ruby
  volumes:
    - ssh:/ssh
  environment:
    SSH_AUTH_SOCK: /ssh/auth/sock

example-session.sh

$ sudo -E ./start-agent-with-key
ssh-agent
Identity added: /keys/id_rsa (/keys/id_rsa)
$ docker-compose run --rm app bash
root@5f914ff271b9:/# ssh-add -l
2048 83:29:38:e9:19:2a:94:68:f6:67:21:78:3a:8a:ed:2a /keys/id_rsa (RSA)

start-agent-with-key.sh

#!/bin/bash
#
# This script adds your SSH key from ~/.ssh/id_rsa into an ssh-agent container.
#
set -euo pipefail

declare SSH_DIR=$PWD/ssh

# Start agent
docker start ssh-agent || docker run -d -v ssh:/ssh --name=ssh-agent whilp/ssh-agent

# Add key from id_rsa
docker run --rm -v ssh:/ssh:ro -v $SSH_DIR:/keys:ro whilp/ssh-agent ssh-add /keys/id_rsa