Configure docker container with SSH access

Scenario

Often you want to make some changes on a virtual machine or remote machine from a pipeline script; this is usually done by using Ansible, but even before that, SSH access to the target machine must be available.

Assuming that you have a main script that prepares a docker container to do the workload, how do you ensure that the docker container has access to a target machine that it wants to access?

Prerequisites

A Linux or MacOS machine for local development. If you are running Windows, you first need to set up the Windows Subsystem for Linux (WSL) environment.

You need docker cli and docker-compose on your machine for testing purposes, and/or on the machines that run your pipeline.
You can check both of these by running the following commands:

docker - version
docker-compose - version

Set the following environment variable for SSH access:
- TARGET_MACHINE_SSH_PRIVATE_KEY
You can name it however you want. For convenience, you can add it directly to your profile. This can look like this, for example:

export TARGET_MACHINE_SSH_PRIVATE_KEY="-----BEGIN OPENSSH PRIVATE KEY-----
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaa
-----END OPENSSH PRIVATE KEY-----"

Note the syntax: it needs double quotation marks at the beginning and the end of the ssh key value.

Note: if you use GitLab, the value of the environment variable must contain the content only, without the double quotation marks.

Implementation

When building the docker image, pass an argument to it to provide the value of the TARGET_MACHINE_SSH_PRIVATE_KEY environment variable:

--build-arg SSH_PRIVATE_KEY="$TARGET_MACHINE_SSH_PRIVATE_KEY"

Add the following in your docker file:

ARG SSH_PRIVATE_KEY
RUN mkdir -p /root/.ssh
RUN echo "$SSH_PRIVATE_KEY" | tr -d '\r' > /root/.ssh/id_rsa && chmod 600 /root/.ssh/id_rsa

This will copy the value of SSH_PRIVATE_KEY to the standard location for SSH keys.

Usage

Navigate to the root of the git repository and run the following command:

sh run.sh 

The following happens:
1) the first command builds the docker image, passing the private key value as an argument and tagging it as sshaccess
2) the docker image sets up the SSH access by copying the value of the SSH_PRIVATE_KEY argument to the standard location for SSH keys
3) the second command uses docker-compose to create and run the container. The container runs an SSH check against a target machine and prints some output to inform whether it was successful or not

GitHub source