title: Gitlab-ci does not recognize python
body: <p>I want to run my Jupyter notebooks with my pipeline. To do this i need <a href="https://pypi.org/project/runipy/" rel="nofollow noreferrer">runipy</a>. And to install runipy on the gitlab runner i need python. But when i want to run the .gitlab-ci.yml script it gets stuck at printing the python version because python is not recognized.</p>
  <p><strong>.gitlab-ci.yml:</strong></p>
  <pre><code>image: python:3.9
  
  before_script:
    - python -V
    - pip install runipy
  </code></pre>
  <p><strong>error:</strong></p>
  <pre><code>Running with gitlab-runner 13.3.1 (738bbe5a)
    on JavaUnitTestRunner vr_SgzNd
  Preparing the &quot;shell&quot; executor
  00:00
  Using Shell executor...
  Preparing environment
  00:01
  Running on DESKTOP-DJVC49J...
  Getting source from Git repository
  00:03
  Fetching changes with git depth set to 50...
  Reinitialized existing Git repository in C:/Gitlab-Runner/builds/vr_SgzNd/0/I438220/pharma/.git/
  Checking out 65a225c0 as pipeline...
  git-lfs/2.9.2 (GitHub; windows amd64; go 1.12.7; git 0274d856)
  Skipping Git submodules setup
  Executing &quot;step_script&quot; stage of the job script
  00:02
  $ python -V
  python : The term 'python' is not recognized as the name of a cmdlet, function, script file, or operable program. Check
   the spelling of the name, or if a path was included, verify that the path is correct and try again.
  At C:\WINDOWS\TEMP\build_script377697220\script.ps1:195 char:1
  + python -V
  + ~~~~~~
      + CategoryInfo          : ObjectNotFound: (python:String) [], CommandNotFoundException
      + FullyQualifiedErrorId : CommandNotFoundException
   
  ERROR: Job failed: exit status 1
  </code></pre>
  <p>When i try the same command in cmd everything works just fine.</p>