title: How to get the short sha for the github workflow?
body: <p>I am creating a workflow in GitHub which creates and uses a docker image. Therefore I have started my workflow file with a global environment variable for this docker image which is visible for all the jobs in my workflow:</p>
  
  <pre><code>name: continuous integration
  on:
    push:
      branches:
        - '**'
  
  env:
    IMAGE: docker.pkg.github.com/${{ github.repository }}/jactor-persistence:${{ github.sha }}
  </code></pre>
  
  <p>I want to replace <code>${{ github.sha }}</code> with the short sha of the head commit, the same as the result of the following command <code>git rev-parse --short HEAD</code></p>
  
  <p>Is this possible?</p>