feat(PROJ-27): Docker-Support + install.sh v2.0 (native + Docker-Modus)

Adds multi-stage Dockerfiles (Go+Xapian CGO, Next.js standalone),
docker-compose.yml, webhook-basierter Deploy-Flow und erweitertes
install.sh mit interaktiver Modus-Auswahl.

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

config/config.docker.yml.example

@@ -0,0 +1,57 @@
+# archivmail Docker-Konfiguration
+# Kopieren nach: /etc/archivmail/config.yml
+# Wird als Read-Only-Volume in den Container gemountet.
+
+server:
+  api_port: 8080
+  smtp_port: 2525
+
+database:
+  host: postgres       # Docker Compose Service-Name (nicht localhost!)
+  port: 5432
+  name: archivmail
+  user: archivmail
+  password: CHANGE_ME_DB_PASSWORD
+  sslmode: disable
+
+storage:
+  store_path:  /var/archivmail/store
+  astore_path: /var/archivmail/astore
+  xapian_path: /var/archivmail/xapian
+  keyfile:     /etc/archivmail/keyfile
+
+index:
+  path:             /var/archivmail/xapian
+  backend:          xapian
+  batch_size:       100
+  async_queue_size: 1000
+
+api:
+  bind:           "0.0.0.0:8080"
+  secret:         CHANGE_ME_64_CHAR_SECRET
+  secure_cookies: true
+  trusted_proxies:
+    - "172.16.0.0/12"   # Docker-interne Netzwerke
+    - "127.0.0.1"
+
+smtp:
+  enabled:           true
+  bind:              "0.0.0.0:2525"
+  domain:            mail.example.com
+  max_size_mb:       25
+  allowed_ips:
+    - "0.0.0.0/0"
+    - "::/0"
+  tenant_routing:    default
+  default_tenant_id: 1
+
+imap_server:
+  enabled: false
+  bind:    "0.0.0.0:1143"
+
+audit:
+  log_path:       /var/archivmail/audit.log
+  retention_days: 365
+
+logging:
+  level: info