Using SSH keys inside docker container

Turns out when using Ubuntu, the ssh_config isn't correct. You need to add

RUN  echo "    IdentityFile ~/.ssh/id_rsa" >> /etc/ssh/ssh_config

to your Dockerfile in order to get it to recognize your ssh key.

Note: only use this approach for images that are private and will always be!

The ssh key remains stored within the image, even if you remove the key in a layer command after adding it (see comments in this post).

In my case this is ok, so this is what I am using:

# Setup for ssh onto github
RUN mkdir -p /root/.ssh
ADD id_rsa /root/.ssh/id_rsa
RUN chmod 700 /root/.ssh/id_rsa
RUN echo "Host github.com\n\tStrictHostKeyChecking no\n" >> /root/.ssh/config

If you are using Docker Compose an easy choice is to forward SSH agent like that:

something:
    container_name: something
    volumes:
        - $SSH_AUTH_SOCK:/ssh-agent # Forward local machine SSH key to docker
    environment:
        SSH_AUTH_SOCK: /ssh-agent

or equivalently, if using docker run:

$ docker run --mount type=bind,source=$SSH_AUTH_SOCK,target=/ssh-agent \
             --env SSH_AUTH_SOCK=/ssh-agent \
             some-image

It's a harder problem if you need to use SSH at build time. For example if you're using git clone, or in my case pip and npm to download from a private repository.

The solution I found is to add your keys using the --build-arg flag. Then you can use the new experimental --squash command (added 1.13) to merge the layers so that the keys are no longer available after removal. Here's my solution:

Build command

$ docker build -t example --build-arg ssh_prv_key="$(cat ~/.ssh/id_rsa)" --build-arg ssh_pub_key="$(cat ~/.ssh/id_rsa.pub)" --squash .

Dockerfile

FROM python:3.6-slim

ARG ssh_prv_key
ARG ssh_pub_key

RUN apt-get update && \
    apt-get install -y \
        git \
        openssh-server \
        libmysqlclient-dev

# Authorize SSH Host
RUN mkdir -p /root/.ssh && \
    chmod 0700 /root/.ssh && \
    ssh-keyscan github.com > /root/.ssh/known_hosts

# Add the keys and set permissions
RUN echo "$ssh_prv_key" > /root/.ssh/id_rsa && \
    echo "$ssh_pub_key" > /root/.ssh/id_rsa.pub && \
    chmod 600 /root/.ssh/id_rsa && \
    chmod 600 /root/.ssh/id_rsa.pub

# Avoid cache purge by adding requirements first
ADD ./requirements.txt /app/requirements.txt

WORKDIR /app/

RUN pip install -r requirements.txt

# Remove SSH keys
RUN rm -rf /root/.ssh/

# Add the rest of the files
ADD . .

CMD python manage.py runserver

---

Update: If you're using Docker 1.13 and have experimental features on you can append --squash to the build command which will merge the layers, removing the SSH keys and hiding them from docker history.