title: Can't add user to docker group
body: <p>I'm trying to set docker up on a new system, and when running docker info I get:</p>
  
  <pre><code>docker -v
  =&gt; Docker version 18.09.5, build e8ff056
  
  docker info
  =&gt; Got permission denied while trying to connect to the Docker daemon
     socket at unix:///var/run/docker.sock: Get
     http://%2Fvar%2Frun%2Fdocker.sock/v1.39/info: dial unix 
     /var/run/docker.sock: connect: permission denied
  </code></pre>
  
  <p>Following the docs, I've tried:</p>
  
  <pre><code>sudo usermod -a -G docker $USER
  </code></pre>
  
  <p>Which returns no output. When I then run groups:</p>
  
  <pre><code>groups
  =&gt; mark adm cdrom sudo dip plugdev lpadmin sambashare
  </code></pre>
  
  <p>I can see a docker group exists:</p>
  
  <pre><code>less /etc/group | grep docker
  =&gt; docker:x:131:mark
  </code></pre>
  
  <p>And can see that it owns a socket running where the error message states:</p>
  
  <pre><code>ls -la /var/run/ | grep docker
  =&gt; 
  drwx------  5 root                root                 120 May 25 14:54 docker
  -rw-r--r--  1 root                root                   5 May 25 14:54 docker.pid
  srw-rw----  1 root                docker                 0 May 25 14:54 docker.sock
  </code></pre>
  
  <p>So why can't I add myself to that group with <code>sudo usermod -a -G docker $USER</code> ?</p>