docs: Add setup section to README (#185)

* Ignore .env

* Create .env.sample

* docs: Add Docker usage instructions to README

Add section explaining how to run the project with Docker Compose,
including examples for different Ruby versions and development server.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* docs: Setup/Build/Serve

* docs: Simplify docker document

* fix: Fix file name

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

.env.sample

@@ -0,0 +1 @@
+RUBY_VERSION=2.7

.gitignore

@@ -1,2 +1,3 @@
 _site
 .sass-cache
+.env

README.md

@@ -1,10 +1,10 @@
-# [railsdoc.github.io](https://railsdoc.github.io/) 
+# [railsdoc.github.io](https://railsdoc.github.io/)
 
 [![CI](https://github.com/railsdoc/railsdoc.github.io/actions/workflows/ci.yml/badge.svg)](https://github.com/railsdoc/railsdoc.github.io/actions/workflows/ci.yml)
 [![Deploy](https://github.com/railsdoc/railsdoc.github.io/actions/workflows/deploy.yml/badge.svg)](https://github.com/railsdoc/railsdoc.github.io/actions/workflows/deploy.yml)
 [![Netlify Status](https://api.netlify.com/api/v1/badges/c964029a-6d5a-4f3a-95e9-d35830a2fe83/deploy-status)](https://app.netlify.com/sites/railsdoc-preview/deploys)
 
-railsdoc.github.io is yet another Rails API documentation website.
+[railsdoc.github.io](https://railsdoc.github.io/) is yet another Rails API documentation website.
 
 ## Project Goal
 
@@ -24,6 +24,45 @@ There are some enhancements in addition to SEO.
 - [ ] Comment System
 - [ ] etc. (see. https://github.com/railsdoc/railsdoc.github.io/issues)
 
-## Feature Request  
+## Setup
+
+```console
+$ 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`.
+
+For specific Rails versions:
+
+```console
+$ rake build_multi[5.2,6.0,6.1]
+```
+
+### Using docker for the older Rails build
+
+Set `RUBY_VERSION` in `.env` file and run `docker compose`.
+
+```console
+$ cp .env.sample .env
+$ docker compose run app bash -c 'rake build_multi[5.2]'
+```
+
+## Serve docs
+
+Serve html under `src` directory.
+
+```console
+$ bundle exec jekyll server
+```
+
+## Feature Request
 
 Please create an [issue](https://github.com/railsdoc/railsdoc.github.io/issues).