switch to official mainsail docker image

* update docs
* update image in compose file
* remove mainsail dockerfile
* remove mainsail image workflow

.github/workflows/mainsail.yaml

@@ -1,13 +0,0 @@
-name: 'Image: Mainsail'
-on:
-  workflow_dispatch:
-  schedule:
-    - cron: '0 2 * * *'
-jobs:
-  build:
-    uses: mkuf/prind/.github/workflows/image-workflow.yaml@main
-    with:
-      app: mainsail
-    secrets:
-      DOCKERHUB_USERNAME: ${{ secrets.DOCKERHUB_USERNAME }}
-      DOCKERHUB_TOKEN: ${{ secrets.DOCKERHUB_TOKEN }}