Docker_image.rst: remove pip install instruction

http://docs.coala.io/en/latest/Users/Docker_Image.html#coala-on-gitlab-ci mentions invokingpip install -r requirements.txt. It is unnecessary and the documentation has been updated to show the changes.

Closes coala#486

Users/Docker_Image.rst

@@ -73,7 +73,6 @@ on your code with a ``.gitlab-ci.yml``, like this:
     check_code:
       image: coala/base
       script:
-      - pip install -r requirements.txt
       - coala --ci
 
 .. note::