title: Using GitLab CI with the Python 'onbuild' image, the packages in requirements.txt don't seem to get installed
body: <p>I'm trying to familiarize myself with the <a href="https://docs.gitlab.com/ce/ci/quick_start/" rel="nofollow noreferrer">Gitlab CI</a> environment with a test project, <a href="https://gitlab.com/khpeek/CI-test" rel="nofollow noreferrer">https://gitlab.com/khpeek/CI-test</a>. The project has the following <code>.gitlab-ci.yml</code>:</p>
  
  <pre><code>image: python:2.7-onbuild
  services:
    - rethinkdb:latest
  test_job:
    script:
      - pytest
  </code></pre>
  
  <p>The problem is that the <code>test_job</code> job in the CI pipeline fails with the following error message:</p>
  
  <pre><code>Running with gitlab-ci-multi-runner 9.0.1 (a3da309)
    on docker-auto-scale (e11ae361)
  Using Docker executor with image python:2.7-onbuild ...
  Starting service rethinkdb:latest ...
  Pulling docker image rethinkdb:latest ...
  Using docker image rethinkdb:latest ID=sha256:23ecfb08823bc5483c6a955b077a9bc82899a0df2f33899b64992345256f22dd for service rethinkdb...
  Waiting for services to be up and running...
  Using docker image sha256:aaecf574604a31dd49a9d4151b11739837e4469df1cf7b558787048ce4ba81aa ID=sha256:aaecf574604a31dd49a9d4151b11739837e4469df1cf7b558787048ce4ba81aa for predefined container...
  Pulling docker image python:2.7-onbuild ...
  Using docker image python:2.7-onbuild ID=sha256:5754a7fac135b9cae7e02e34cc7ba941f03a33fb00cf31f12fbb71b8d389ece2 for build container...
  Running on runner-e11ae361-project-3083420-concurrent-0 via runner-e11ae361-machine-1491819341-82630004-digital-ocean-2gb...
  Cloning repository...
  Cloning into '/builds/khpeek/CI-test'...
  Checking out d0937f33 as master...
  Skipping Git submodules setup
  $ pytest
  /bin/bash: line 56: pytest: command not found
  ERROR: Job failed: exit code 1
  </code></pre>
  
  <p>However, there is a <code>requirements.txt</code> in the repository with the single line <code>pytest==3.0.7</code> in it. It seems to me from the <a href="https://github.com/docker-library/python/blob/7663560df7547e69d13b1b548675502f4e0917d1/2.7/onbuild/Dockerfile" rel="nofollow noreferrer">Dockerfile</a> of the <code>python:2.7-onbuild</code> image, however, that <code>pip install -r requirements.txt</code> should get run on build. So why is <code>pytest</code> not found?</p>