Merge branch 'patch-1' into 'master'

Fixed "Docker Fundamentals" link in GitLab Documentation.

I noticed that the *Docker Fundamentals* link in the *Using Docker Images* part of the GitLab Documentation is broken.

Screenshot:
![broken-link](/uploads/8d2fc782b2f5d4f3316a631f801cd3c7/broken-link.png)

Changing it to "https://docs.docker.com/" should fix the problem.

See merge request !2786

doc/ci/docker/using_docker_images.md

@@ -270,7 +270,7 @@ This will forcefully (`-f`) remove the `build` container, the two service
 containers as well as all volumes (`-v`) that were created with the container
 creation.
 
-[Docker Fundamentals]: https://docs.docker.com/engine/introduction/understanding-docker/
+[Docker Fundamentals]: https://docs.docker.com/engine/understanding-docker/
 [hub]: https://hub.docker.com/
 [linking-containers]: https://docs.docker.com/engine/userguide/networking/default_network/dockerlinks/
 [tutum/wordpress]: https://registry.hub.docker.com/u/tutum/wordpress/