Add Docker setup guide for application templates

[Copilot]

No guide existed for the Docker infrastructure bundled with yiisoft/app and yiisoft/app-api. Both templates ship with a FrankenPHP/Caddy-based setup covering dev, test, and production environments, but it was undocumented.

Changes

• New page src/guide/tutorial/docker.md covering:

  • Annotated docker/ directory tree
  • All four .env files (docker/.env, docker/dev/.env, docker/test/.env, docker/prod/.env) with per-variable tables distinguishing Compose-level from app-level variables
  • Override env file pattern (override.env.example) for developer-local settings
  • Each compose.yml (base, dev, test, prod/Swarm) with key config points explained
  • Multi-stage Dockerfile breakdown: composer → base → dev / prod-builder → prod
  • Full make command reference grouped by Development / Tools / Testing / Production

• src/.vitepress/config.js — added page to Tutorial sidebar

• src/guide/index.md — added link under Special topics

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create a guide page about Docker setup in application templates</issue_title>
<issue_description>### Description

Create a user guide for Docker in https://github.com/yiisoft/app and https://github.com/yiisoft/app-api

Things to note especially:

1. make commands usage.
2. Contents of /docker overview.
3. Each .env purpose and examples.
4. compose.yml.
5. The idea of multi-stage builds.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Create a guide page about Docker setup in application templates #390

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