Use Gitea registry image with env-based configuration

Replace local build with configurable registry image defaults and document
settings in .env.example for deployers.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -0,0 +1,19 @@
+# Image (Gitea container registry)
+IMAGE_REGISTRY=git.nxtgroup.org
+IMAGE_OWNER=erling
+IMAGE_NAME=btcpaymailer
+IMAGE_TAG=1.0.0
+
+# Container
+CONTAINER_NAME=btcpay-mailer
+HOST_PORT=5000
+
+# Required secrets
+WEBHOOK_SECRET=your_super_secret_token_123
+BTCPAY_API_KEY=your_btcpay_api_key
+POSTMARK_API_KEY=your_postmark_server_token
+
+# Optional
+BTCPAY_URL=https://payment.nxtgroup.org
+FROM_EMAIL=billing@nxtgroup.org
+BCC_EMAIL=admin@nxtgroup.org,finance@nxtgroup.org

docker-compose.yml

@@ -1,15 +1,16 @@
 version: '3.8'
+
 services:
   btcpay-mailer:
-    build: .
-    container_name: btcpay-mailer
+    image: ${IMAGE_REGISTRY:-git.nxtgroup.org}/${IMAGE_OWNER:-erling}/${IMAGE_NAME:-btcpaymailer}:${IMAGE_TAG:-1.0.0}
+    container_name: ${CONTAINER_NAME:-btcpay-mailer}
     restart: unless-stopped
     ports:
-      - "5000:5000"
+      - "${HOST_PORT:-5000}:5000"
     environment:
-      - WEBHOOK_SECRET=your_super_secret_token_123
-      - BTCPAY_URL=https://payment.nxtgroup.org
-      - BTCPAY_API_KEY=your_btcpay_api_key
-      - POSTMARK_API_KEY=your_postmark_server_token
-      - FROM_EMAIL=billing@nxtgroup.org
-      - BCC_EMAIL=noreply@nxtgroup.org
+      WEBHOOK_SECRET: ${WEBHOOK_SECRET:?Set WEBHOOK_SECRET in .env or environment}
+      BTCPAY_URL: ${BTCPAY_URL:-https://payment.nxtgroup.org}
+      BTCPAY_API_KEY: ${BTCPAY_API_KEY:?Set BTCPAY_API_KEY in .env or environment}
+      POSTMARK_API_KEY: ${POSTMARK_API_KEY:?Set POSTMARK_API_KEY in .env or environment}
+      FROM_EMAIL: ${FROM_EMAIL:-billing@nxtgroup.org}
+      BCC_EMAIL: ${BCC_EMAIL:-admin@nxtgroup.org}