Merge branch 'gitlab-ci-ssh-keys-doc-fix' into 'master'

Documentation fix in CI SSH keys setup

In recent versions of Docker, the `/.dockerinit` file doesn't exist any more when running inside a container. That file is used in the example here to detect whether the runner is running inside Docker or not. Change it to check for the existence of `/.dockerenv`, which does exist in all current versions of Docker.

See merge request !3938

doc/ci/ssh_keys/README.md

@@ -57,7 +57,7 @@ before_script:
   # WARNING: Use this only with the Docker executor, if you use it with shell
   # you will overwrite your user's SSH config.
   - mkdir -p ~/.ssh
-  - '[[ -f /.dockerinit ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
+  - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
 ```
 
 As a final step, add the _public_ key from the one you created earlier to the