title: gitlab runner: mysqld: Can't read dir of '/etc/mysql/conf.d/'
body: <p>I use gitlab runners in docker.</p>
  
  <p>It uses our own php image and mysql:5.7 image. It worked before, but now we can see this error when CI job is started:</p>
  
  <pre><code>Running with gitlab-runner 10.4.0 (857480b6)
    on gitlab-runner-2 (00929f5e)
  Using Docker executor with image registry.blahblah.work/infra/docker-base/php ...
  Starting service mysql:5.7 ...
  Pulling docker image mysql:5.7 ...
  Using docker image mysql:5.7 ID=sha256:f008d8ff927dc527c5a57251b45cead7c9259c16a6a93c144f397eaafc103d36 for mysql service...
  Waiting for services to be up and running...
  
  *** WARNING: Service runner-00929f5e-project-4-concurrent-0-mysql-0 probably didn't start properly.
  
  Error response from daemon: Cannot link to a non running container: /runner-00929f5e-project-4-concurrent-0-mysql-0 AS /runner-00929f5e-project-4-concurrent-0-mysql-0-wait-for-service/runner-00929f5e-project-4-concurrent-0-mysql-0
  
  2018-02-13T08:49:03.752410664Z 
  2018-02-13T08:49:03.752468595Z ERROR: mysqld failed while attempting to check config
  2018-02-13T08:49:03.752476292Z command was: "mysqld --verbose --help"
  2018-02-13T08:49:03.752481759Z 
  2018-02-13T08:49:03.752486693Z mysqld: Can't read dir of '/etc/mysql/conf.d/' (Errcode: 13 - Permission denied)
  2018-02-13T08:49:03.752491885Z mysqld: [ERROR] Fatal error in defaults handling. Program aborted!
  </code></pre>
  
  <p>Variables are set:</p>
  
  <pre><code>variables:
    MYSQL_DATABASE: test
    MYSQL_ROOT_PASSWORD: password
  </code></pre>
  
  <p>What should I do to debug and fix this issue?</p>