docs: add Docker Compose environments guide and fix compose files

- New deploy/ENVIRONMENTS.md: comprehensive walkthrough of all 4 compose
  files with service-by-service explanations, beginner-friendly Docker
  concepts, and expert-level networking/config details
- Fix docker-compose.dev.yml: agent LOG_LEVEL → CERTCTL_LOG_LEVEL (was
  silently ignored without the CERTCTL_ prefix)
- Add CERTCTL_CONFIG_ENCRYPTION_KEY to base and test compose (enables
  M34/M35 dynamic issuer/target config encryption)
- Add CERTCTL_DISCOVERY_DIRS to base compose agent (enables filesystem
  certificate discovery in default deployment)
- Cross-link ENVIRONMENTS.md from README doc table and quickstart.md

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

deploy/docker-compose.yml

@@ -48,6 +48,7 @@ services:
       CERTCTL_AUTH_TYPE: none
       CERTCTL_KEYGEN_MODE: server  # Demo uses server-side keygen; production should use "agent"
       CERTCTL_NETWORK_SCAN_ENABLED: "true"  # Enable network scan GUI with seeded demo targets
+      CERTCTL_CONFIG_ENCRYPTION_KEY: ${CERTCTL_CONFIG_ENCRYPTION_KEY:-change-me-32-char-encryption-key}  # AES-256-GCM for dynamic issuer/target config
     ports:
       - "8443:8443"
     networks:
@@ -83,6 +84,7 @@ services:
       CERTCTL_API_KEY: ${CERTCTL_API_KEY:-change-me-in-production}
       CERTCTL_AGENT_NAME: docker-agent
       CERTCTL_LOG_LEVEL: info
+      CERTCTL_DISCOVERY_DIRS: /var/lib/certctl/keys  # Agent scans this directory for existing certificates
     volumes:
       - agent_keys:/var/lib/certctl/keys
     networks: