docker permission denied error

While connecting from jenkins plugin: Docker plugin, at cloud section of system cofiguration.
When we adding docker details and click on test, sometimes we get permission denied error.

Solution:
Add user :  jenkins, created during jenkins installation, to the root group(not standard) or docker group.

Reference

To list all users:
getent passwd


The user jenkins needs to be added to the group docker:
sudo usermod -a -G docker jenkins

reference