feat: add msmtp to Netdata for email alerts

Created custom Dockerfile for Netdata container that includes msmtp
and msmtp-mta packages for sending email notifications.

Changes:
- Added netdata/Dockerfile extending netdata/netdata:latest
- Installed msmtp, msmtp-mta, and ca-certificates via apk
- Set proper permissions (600) for msmtprc config file
- Updated compose.yaml to build from custom Dockerfile

This enables Netdata to send health alarm notifications via email
using the IONOS SMTP configuration in msmtprc.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

netdata/compose.yaml

@@ -1,5 +1,8 @@
 services:
   netdata:
+    build:
+      context: .
+      dockerfile: Dockerfile
     image: ${NETDATA_IMAGE:-netdata/netdata:latest}
     container_name: ${NETDATA_COMPOSE_PROJECT_NAME}_app
     restart: unless-stopped