Digger supports drift detection alerting via Slack. To configure drift detection:

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Create a separate workflow file for drift

To run digger in drift detection mode, pass mode: drift-detection in the workflow file and configure the relevant crontab to run it with the frequency you want: 
name: Digger Drift Detection

on:  
  workflow_dispatch:
  schedule: ## 12am daily.
    - cron: '0 0 * * *'

jobs:
  detect-drift:
    runs-on: ubuntu-latest
    steps:
    - name: digger drift detection
      uses: diggerhq/digger@latest
      with:
        mode: drift-detection
        setup-aws: true
        aws-access-key-id: ${{ secrets.AWS_ACCESS_KEY_ID }}
        aws-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
        aws-region: us-east-1
        digger-token: ${{ secrets.DIGGER_TOKEN }}
        digger-organisation: digger
        digger-hostname: https://cloud.digger.dev/
        drift-detection-slack-notification-url: ${{ secrets.DRIFT_DETECTION_SLACK_NOTIFICATION }}
      env:
        GITHUB_CONTEXT: ${{ toJson(github) }}
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

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Configure Slack notification URL

Note the DRIFT_DETECTION_SLACK_NOTIFICATION env var that the workflow above is using. This should be set to a Slack Incoming Webhook URL. Follow the official Slack guide to get the Incoming Webhook URL; then add it as an Action secret named DRIFT_DETECTION_SLACK_NOTIFICATION