A simple host based step for github actions to post to slack, no docker needed

github-action-step-post-slack.yaml

    - name: Build failure
      if: failure()
      env:
        SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
        SLACK_CHANNEL: ...
      run: |
        curl -X POST \
            -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
            -d channel=$SLACK_CHANNEL \
            -d text='[${{ github.repository }}] ${{ github.event.ref }} *FAILED*' \
            https://slack.com/api/chat.postMessage