DEV: Overhaul devcontainer configuration (#28446)

- Uses a more appropriate image, with immutable tag (so update prompts work correctly)
- Updates port forwarding
- Improves mount setup (inc. persistant PG/Redis when rebuilding)
- Fixes ember-cli live reload
- Automatically configures VSCode & extensions

.devcontainer/scripts/start.rb

@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+puts "👋 Welcome to the Discourse devcontainer! Let's get everything ready..."
+
+puts "Setting permissions on volume mounts..."
+system "sudo chown discourse .", exception: true
+system "sudo chown discourse node_modules", exception: true
+system "sudo chown -R postgres /shared/postgres_data", exception: true
+
+puts "Starting services..."
+fork do
+  Process.daemon
+  exec "sudo nohup /sbin/boot"
+end
+
+system "cp -n .vscode/settings.json.sample .vscode/settings.json", exception: true
+system "cp -n .vscode/tasks.json.sample .vscode/tasks.json", exception: true
+
+puts <<~TXT
+  🎉 All done!
+
+  Next steps:
+    1. Cmd/Ctrl + Shift + B to run the shortcuts/boot-dev task
+    2. Wait for the server to start
+    3. Open your browser to http://localhost:4200
+TXT