docker docker-compose dockerfile ssh-keys docker-image

SSH agent forwarding during docker build

While building up a docker image through a dockerfile, I have to clone a github repo. I added my public ssh keys to my git hub account and I am able to clone the repo from my docker host. While I see that I can use docker host's ssh key by mapping $SSH_AUTH_SOCK env variable at the time of docker run like

docker run --rm -it --name container_name \
  -v $(dirname $SSH_AUTH_SOCK):$(dirname $SSH_AUTH_SOCK) \
  -e SSH_AUTH_SOCK=$SSH_AUTH_SOCK my_image

How can I do the same during a docker build?

Solution

For Docker 18.09 and newer

You can use new features of Docker to forward your existing SSH agent connection or a key to the builder. This enables for example to clone your private repositories during build.

Steps:

First set environment variable to use new BuildKit

export DOCKER_BUILDKIT=1

Then create Dockerfile with new (experimental) syntax:

# syntax=docker/dockerfile:experimental

FROM alpine

# install ssh client and git
RUN apk add --no-cache openssh-client git

# download public key for github.com
RUN mkdir -p -m 0600 ~/.ssh && ssh-keyscan github.com >> ~/.ssh/known_hosts

# clone our private repository
RUN --mount=type=ssh git clone git@github.com:myorg/myproject.git myproject

And build image with

docker build --ssh default .