fix: in recent versions of Docker, the /.dockerinit file doesn't exist; use…

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