'docker ps' responded with both the 'permission denied' message and the actual message

Question

I'm sorry if this seems to be a stupid question since I'm completely new to Jenkins and Docker. I'm setting up Jenkins and Docker in an Azure linux(Ubuntu 16.04) VM. Since I want to run docker in Jenkinsfile (without sudo), I will need to add user to docker group.
Therefore I followed this: Post-installation steps for Linux from Docker. However, I'm getting this error message when running $ sudo usermod -aG docker $USER

sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting

After restarting the VM, I'm now getting this message after running $ Docker images

WARNING: Error loading config file: /home/terryxychan/.docker/config.json: stat /home/terryxychan/.docker/config.json: permission denied
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
hello-world         latest              fce289e99eb9        2 months ago        1.84k

Somehow I'm getting both the 'permission denied' message and the actual message itself.

When I tried to build it using jenkinsfile, it gives me this error:

+ docker build -t terryxychan/python-demo-1:4 .
Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock

Does anyone ran into the same problem with Azure VM before? Thanks!!!