feat(mysql): add MySQL 8 Alpine compose file

- Add docker-compose.yaml with MYSQL_ROOT_PASSWORD, MYSQL_DATABASE,
  MYSQL_USER, and MYSQL_PASSWORD environment variables
- Add .env.example with documented defaults

compose/mysql/.env.example

@@ -0,0 +1,16 @@
+# MySQL (Alpine) — environment variables
+# Copy this file to .env and adjust values for your environment.
+
+# Root password.
+# The default is for local development only — always change for production.
+MYSQL_ROOT_PASSWORD=mysql
+
+# Database created automatically on first start (default: mysql).
+MYSQL_DATABASE=mysql
+
+# Additional user created automatically on first start (default: root).
+MYSQL_USER=root
+
+# Password for the additional user.
+# The default is for local development only — always change for production.
+MYSQL_PASSWORD=mysql