Docker_image.rst: remove pip install instruction

`pip install` is unnecessary in Docker Image docs.

Closes coala#486

Author: raghavgr

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::