Slack notification Concourse resource v1.8.0

Improvements

• Have the resource fail when a message has faild to be posted on some channel (originally submitted in #97, thanks @Jazzepi)
• Built resource image on top of latest Alpine v3.20.3
• Improved code readability, for easier maintenance
• Improved Concourse workflow, with continuous tests on the develop branch

Usage

resource_types:
  - name: notify-slack
    type: registry-image
    source:
      repository: cfcommunity/slack-notification-resource
      tag:        "1.8.0"