Pass ssh-agent to dockerfile to install private repository modules - docker

Question

I am trying to automate a docker build in Jenkins pipeline. In my dockerfile, I basically build a node application. In my npm install, I have some private git repositories which need os bindings and so have to be installed in the container. When I run this manually, I transfer my ssh keys (id_rsa) to dockerfile which is used for doing npm install. Now, my problem is when running this task in jenkins pipeline, I will be configuring a ssh-agent(Jenkins plugin). It will not be possible to extract private key from ssh-agent. How should I pass my ssh-agent to my dockerfile.
EDIT 1:
I got it partially working by this:
Docker Build Command:
DOCKER_BUILDKIT=1 docker build --no-cache -t $DOCKER_REGISTRY_URL/$IMAGE_NAME:v$BUILD_NUMBER --ssh default . &&
Then in Docker file:
This works fine:
RUN --mount=type=ssh GIT_SSH_COMMAND="ssh -vvvT -o StrictHostKeyChecking=no"
git clone git#github.com:****
Weird thing is this doesn't work:
RUN --mount=type=ssh GIT_SSH_COMMAND="ssh -vvvT -o StrictHostKeyChecking=no" npm install git+ssh//git#github.com:****
I feel this is something to do with StrictHostKeyChecking=no

Answer 1

I finally got it working by using ROOT user in Dockerfile and setting the npm cache to root.
The problem was that git was using the /root/.ssh folder while npm was using a different path - /home/.ssh as it's npm cache was set on /home/.ssh
For anyone still struggling, this is the config I used
Docker Build Command:
DOCKER_BUILDKIT=1 docker build --no-cache -t test --ssh default .
Dockerfile:
USER root
RUN apt-get update && \
apt-get install -y \
git \
openssh-server \
openssh-client
RUN mkdir -p -m 600 /root/.ssh && ssh-keyscan github.com >> /root/.ssh/known_hosts && echo "Host *\n StrictHostKeyChecking no" > /root/.ssh/config
RUN echo "Check ssh_config" && cat /root/.ssh/config
RUN rm -rf node_modules
RUN npm config set cache /root
RUN --mount=type=ssh GIT_SSH_COMMAND="ssh -vvvT" npm install