SSH key forwarding into docker container pipeline

[Lorem Ipsum]

Hello

I'm trying to build a project which Makefile pulls modules from a private repository. How should I forward the SSH key from Jenkins to the docker container?

The pipeline which creates the build is pretty very basic but it fails on checkout: 

pipeline {
    agent { dockerfile true }
    stages {
        stage('Build'){
            steps {
                sh 'make'
            }
        }
    }
}

I assume that similar problem will occur for known_hosts entry for the git repo. Are there any best practices for this? 

[Lorem Ipsum]

I've tried modifying the Jenkinsfile to the following: 

pipeline {
    agent { dockerfile true }
    stages {
        stage('Build'){
            steps {


                sshagent(['d288a45e-5b02-4aac-839b-d87cc3a854d0']) {
                    sh 'make'
                }
            }
        }
    }
}

But the checkout fails with the following error:

No user exists for uid 112
fatal: Could not read from remote repository.

Should I specially prepare the docker for Jenkins to use?