title: apt not found when I use apt in gitlab ci before_script
body: <p>I use gitlab ci to build docker image and I want to install python. When I build, the following is my gitlab-ci.yml:</p>
  
  <pre><code>image: docker:stable
  stages:
    - test
    - build
  
  before-script:
    - apt install -y python-dev python pip
  
  test1:
    stage: test
    script:
    ...
      - pytest
  
  build:
    stage: build
    - docker build -t $IMAGE_TAG .
    - docker push $IMAGE_TAG
  </code></pre>
  
  <p>but i got a Job failed</p>
  
  <pre><code>/bin/sh: eval: line : apt: not found
  ERROR: Job failed: exit code 127
  </code></pre>
  
  <p>I also tried to apt-get install but the result is the same.</p>
  
  <p>How do I install python??</p>