fix: use apt instead of apk for Netdata Dockerfile

Netdata container is Debian-based, not Alpine. Updated package
manager from apk to apt-get for installing msmtp packages.

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

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

netdata/Dockerfile

@@ -2,7 +2,10 @@
 FROM netdata/netdata:latest
 
 # Install msmtp for sending emails
-RUN apk add --no-cache msmtp msmtp-mta ca-certificates
+RUN apt-get update && \
+    apt-get install -y msmtp msmtp-mta ca-certificates && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
 
 # Set proper permissions for msmtp config
 RUN chmod 600 /etc/msmtprc || true