Allow setting description for a build

[qnm]

Solving #2 for me, and based on this convox/action-deploy@805c876 this PR change can be used like so:

    - name: capture branch
      id: vars
      run: |
        echo ::set-output name=branch::${GITHUB_REF#refs/*/}
        echo ::set-output name=sha_short::$(git rev-parse --short HEAD)
        echo ::set-output name=message::$(git log -1 --pretty=%B)
    - name: production
      uses: digivizer/action-build@description-input
      with:
        rack: production
        app: ${{ github.event.repository.name }}
        description: push digivizer/${{ github.event.repository.name }}:${{ steps.vars.outputs.branch }} ${{ steps.vars.outputs.sha_short }} ${{ steps.vars.outputs.message }}