feat(06-03): create infrastructure for Lab 05 Database & RDS

Infrastructure:
- docker-compose.yml: PostgreSQL in private network (RDS simulation)
- Dockerfile: Alpine-based test image with postgresql-client

Services:
- app: nginx for testing database connection (multi-homed)
- db: PostgreSQL 16 in private network (simulates RDS)
- test-public: Alpine for isolation testing

Key Features:
- Private network with --internal flag (INF-02 compliant)
- Named volume for data persistence (INF-04)
- Resource limits: 2 vCPU, 4GB RAM (INF-03)
- Non-root execution (INF-01)
- NO ports exposed from database

Parallels:
- PostgreSQL container → RDS Instance
- Private network → VPC Private Subnet
- Named volume → EBS volume
- Resource limits → DB instance class

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

labs/lab-05-database/Dockerfile

@@ -0,0 +1,26 @@
+# Dockerfile per Lab 05 - Database & RDS
+# Nota: Lab 05 usa immagini ufficiali (PostgreSQL, Nginx, Alpine)
+# Questo Dockerfile è fornito come reference per customizzazioni future
+
+FROM alpine:3.19
+
+# Creare utente non-root per sicurezza (INF-01 compliance)
+RUN addgroup -g 1000 appgroup && \
+    adduser -D -u 1000 -G appgroup appuser
+
+# Installare strumenti di test database
+RUN apk add --no-cache \
+    postgresql-client \
+    curl \
+    netcat-openbsd \
+    bind-tools \
+    && rm -rf /var/cache/apk/*
+
+# Passare all'utente non-root
+USER appuser
+
+# Set working directory
+WORKDIR /home/appuser
+
+# Comando di default - container in attesa per testing
+CMD ["sh", "-c", "sleep 3600"]