Updates dev container and adds doc for getting started with GitHub Codespaces (#23872)

.devcontainer/docker-compose.yml

@@ -5,15 +5,8 @@ services:
     build:
       context: .
       dockerfile: Dockerfile
-      args:
-        # Update 'VARIANT' to pick a version of Ruby: 3, 3.1, 3.0, 2, 2.7, 2.6
-        # Append -bullseye or -buster to pin to an OS version.
-        # Use -bullseye variants on local arm64/Apple Silicon.
-        VARIANT: '3.0-bullseye'
-        # Optional Node.js version to install
-        NODE_VERSION: '16'
     volumes:
-      - ..:/mastodon:cached
+      - ../..:/workspaces:cached
     environment:
       RAILS_ENV: development
       NODE_ENV: development
@@ -33,7 +26,6 @@ services:
     networks:
       - external_network
       - internal_network
-    user: vscode
 
   db:
     image: postgres:14-alpine