Slack notification Concourse resource v1.7.1

Improvements

• Fixed link to Slack docs in README (originally #93, then merged with #101, thanks @markrmullan!)
• Fixed Concourse pipeline so that Alpine version is properly controlled from the pipeline resources, and version bumping is solid
• Built resource image on top of Alpine 3.19.1

Usage

resource_types:
  - name: notify-slack
    type: registry-image
    source:
      repository: cfcommunity/slack-notification-resource
      tag:        "1.7.1"