refactor: move dockerfiles to package folders

.github/workflows/scheduled_tasks.yml

@@ -5,7 +5,6 @@ on:
     branches: [main]
     paths:
       - 'scheduled_tasks/**'
-      - 'Dockerfile.scheduled_tasks'
 
 jobs:
   deploy:
@@ -25,7 +24,7 @@ jobs:
         with:
           # https://github.com/docker/build-push-action/issues/378
           context: .
-          file: Dockerfile.scheduled_tasks
+          file: scheduled_tasks/Dockerfile
           push: true
           tags: |
             ghcr.io/pluralkit/scheduled_tasks:${{ env.BRANCH_NAME }}