Dockerizing a ruby on rails application in which you have mounted a Rails Engine or include private repository in Gemfile is a bit tricky.

Bundle install on Gemfile needs ssh access to access the engine or private repository.

So, while creating docker image need to pass ssh access keys to docker container and for that you need to follow the following process:

First of all, you need to add below comment on the top of the Dockerfile:

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# syntax=docker/dockerfile:experimental

Note: As adding ssh access to docker container may change in future revisions

Next step is to pass ssh private key 🔑 and public key 🔑 to docker build arguments

If building docker image in CI pipeline then pass from configuration of CI. Two ways to pass ssh keys Docker builld command:

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echo "export DOCKER_BUILD_EXTRAARGS='--ssh default
--build-arg ssh_prv_key="$(cat ~/.ssh/id_rsa)" 
--build-arg ssh_pub_key="$(cat ~/.ssh/id_rsa.pub)"'"

and then call docker build as:

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env DOCKER_BUILDKIT=1 DOCKER_BUILD_EXTRAARGS=$DOCKER_BUILD_EXTRAARGS docker-build -f ${CONFIG}"

Note:- we have used ROK8S configuration

—ssh default which will pass the default ssh keys to the docker which will forward your existing SSH agent connection or a key to the builder. This enables for example to clone your private repositories during build but sometimes it doesnt so it is safe to pass them explicitly

OR pass them to the docker build

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 env DOCKER_BUILDKIT=1 docker build --ssh default \
          --build-arg ssh_prv_key="$(cat ~/.ssh/id_rsa)" \
          --build-arg ssh_pub_key="$(cat ~/.ssh/id_rsa.pub)" \

Docker buildkit is the next generation container image builder

Add ssh keys to Docker builder as follows :

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RUN mkdir -p -m 0700 ~/.ssh && ssh-keyscan github.com >> ~/.ssh/known_hosts
ssh-keyscan(gather ssh public keys)

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

This command will put ssh public 🔑 and private 🔑 in your ssh config.

Now the tricky part while doing bundle install in dockerfile needs some changes

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RUN --mount=type=ssh bundle install

By which it will use the ssh keys we provided here and then it can access the Engine or private repository. 🎉 🥳