fix: update docker-compose to docker compose and add ADMIN_PASSWORD to .env.example (#189)

- Replace deprecated docker-compose (v1) with docker compose (v2) in README
- Add missing ADMIN_PASSWORD to .env.example so new users don't get a
  blank-string warning and can actually log in after first setup

README.md

@@ -83,7 +83,7 @@ cp .env.example .env
 nano .env
 
 # Start with Docker Compose
-docker-compose up -d
+docker compose up -d
 
 # Access at http://localhost:3005
 ```
@@ -128,8 +128,8 @@ PICPEAK_CHANNEL=v2.3.0
 Then update your containers:
 
 ```bash
-docker-compose -f docker-compose.production.yml pull
-docker-compose -f docker-compose.production.yml up -d
+docker compose -f docker-compose.production.yml pull
+docker compose -f docker-compose.production.yml up -d
 ```
 
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