docs: Simplify docker document

Author: toshimaru

README.md

@@ -32,39 +32,36 @@ $ bundle install
 
 ## Build docs
 
+For the latest Rails version:
+
 ```console
 $ rake build
 ```
 
 This command generates Rails docs with `default_rails_version` defined in `_config.yml`.
 
-## Serve docs
-
-Serve html under `src` directory.
+For specific Rails versions:
 
 ```console
-$ bundle exec jekyll server
+$ rake build_multi[5.2,6.0,6.1]
 ```
 
-## Docker Usage
+### Using docker for the older Rails build
 
-You can run this project using Docker Compose:
+Set `RUBY_VERSION` in `.env` file and run `docker compose`.
 
-```bash
-# Run the application
-docker compose run --rm app bundle exec rake
+```console
+$ cp .env.example .env
+$ docker compose run app bash -c 'rake build_multi[5.2]'
+```
 
-# Run with a specific Ruby version
-RUBY_VERSION=3.3 docker compose run --rm app bundle exec rake
+## Serve docs
 
-# Run Jekyll server for development
-docker compose run --rm --service-ports app bundle exec jekyll serve --host 0.0.0.0
-```
+Serve html under `src` directory.
 
-The Docker setup uses the official Ruby image and includes:
-- Ruby with YJIT enabled for better performance
-- Automatic bundle installation
-- Volume mounting for live development
+```console
+$ bundle exec jekyll server
+```
 
 ## Feature Request