Why might the Jenkins user not have permission to access the Docker unix socket?
Solution 1:
I think giving jenkins group privileges to docker unix socket solves the issue. This can be modified with configuring docker daemon startup options in configuration file by adding this line
DOCKER_OPTS=' -G jenkins'
In ubuntu /etc/default/docker is the docker configuration file.
Solution 2:
Run groups command using jenkins. Do you see a docker group? If not, try to reboot that Jenkins slave. Or just kill the Jenkins slave.jar process: ps aux|grep jenkins