Docker in Docker permissions error

You need to map the gid of the docker group on your host to the gid of a group that jenkins belongs to inside your container. Here's a sample from my Dockerfile of how I've built a jenkins slave image:

ARG DOCKER_GID=993

RUN groupadd -g ${DOCKER_GID} docker \
  && curl -sSL https://get.docker.com/ | sh \
  && apt-get -q autoremove \
  && apt-get -q clean -y \
  && rm -rf /var/lib/apt/lists/* /var/cache/apt/*.bin 

RUN useradd -m -d /home/jenkins -s /bin/sh jenkins \
  && usermod -aG docker jenkins

The 993 happens to be the gid of docker on the host in this example, you'd adjust that to match your environment.

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Solution from the OP: If rebuilding isn't a possibility you can set the docker group accordingly in using root and add the user. If you tried this before you may have to delete the group on the slave (groupdel docker):

docker exec -it -u root myjenkins bash
container $ groupadd -g 993 docker
container $ usermod -aG docker jenkins

Personally, i've just had to do this :

sudo gpasswd -a $USER docker 

And it worked out

As mentioned in other answers, you must ensure that user "jenkins" inside the container has permission to issue Docker commands to the Docker engine on the host via the /var/run/docker.sock mount.

An easy way to do this is:

$ docker run --rm -d --group-add $(stat -c '%g' /var/run/docker.sock) -v /var/run/docker.sock:/var/run/docker.sock -P myjenkins

But beware:

If your Jenkins job has the need to execute Docker build/run commands (e.g., to build a container), this is not sufficient. The reason is that the containerized Jenkins will ask the Docker in the host to deploy a Docker agent container and that Docker agent container will face similar "permission errors" when connecting to the Docker daemon on the host.

There is a blog post describing the problem and solution for this at https://blog.nestybox.com/2019/09/29/jenkins.html.