Jenkins pipeline: permission denied on the Docker daemon socket

Add the agent user to the docker group with sudo usermod -a -G docker jenkins, then restart the Jenkins agent process, because group membership only takes effect in new sessions. Verify with the groups command or by checking the docker line in /etc/group. If the agent itself runs in a container with the socket mounted, make sure the container user is in the docker group as well.

Context: Stack Overflow #47854463 (accepted answer, 642 votes): a pipeline step using docker.image(...).inside fails with Got permission denied while trying to connect to the Docker daemon socket. The agent process runs as the jenkins user, which is not a member of the docker group, so it cannot talk to the daemon.