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.dev.yml

@@ -11,9 +11,9 @@ services:
       dockerfile: Dockerfile
     environment:
       # Verbose logging for development
-      LOG_LEVEL: debug
-      SERVER_HOST: 0.0.0.0
-      SERVER_PORT: 8443
+      CERTCTL_LOG_LEVEL: debug
+      CERTCTL_SERVER_HOST: 0.0.0.0
+      CERTCTL_SERVER_PORT: "8443"
     volumes:
       # Mount local source for hot reload (requires air or similar)
       # Uncomment if using air or similar for hot reload:
@@ -30,7 +30,7 @@ services:
       context: ..
       dockerfile: Dockerfile.agent
     environment:
-      LOG_LEVEL: debug
+      CERTCTL_LOG_LEVEL: debug
 
   # PgAdmin for database exploration
   pgadmin: