private-git-in-container-build

.ssh-slash-config

Host github.com
  Hostname github.com
  User git
  IdentityFile ~/.ssh/inside-docker-container

Dockerfile

# syntax=docker/dockerfile:experimental                                         
FROM centos:centos7 AS base                                                     
                                                                                
RUN yum -y updateinfo \                                                         
 && yum -y install git                                                          
                                                                                
RUN mkdir -p -m 0600 ~/.ssh \                                                   
 && ssh-keyscan github.com >> ~/.ssh/known_hosts                                
                                                                                
RUN --mount=type=ssh git clone git@github.com:nballenger/fictional-goggles.git

terminal_session.txt

echo "create a new ssh key"
ssh-keygen -t rsa -b 4096 -C "you@your.domain.tld" -f ~/.ssh/inside-docker-container
echo "Copy ~/.ssh/inside-docker-container.pub into github as a new ssh key"
echo "Add the key file to your ssh agent"
ssh-add ~/.ssh/inside-docker-container
echo "Create the .ssh/config entry and the Dockerfile"
echo "cd to the Dockerfile's enclosing folder"
echo "build, with buildkit enabled by flag, so you get the --ssh option"
DOCKER_BUILDKIT=1 docker build -t centos-git --ssh default .