Add host.docker.internal DNS resolution for containerized gateway

[Copilot]

The gateway container cannot resolve host.docker.internal on Linux, preventing connections to HTTP MCP servers running on the host.

Changes

• run_containerized.sh: Added configure_host_dns() function that detects host IP and adds /etc/hosts entry
  • Tries multiple detection methods with fallbacks: primary interface → default gateway → docker0 bridge → 172.17.0.1
  • Called during startup after validation, before server launch
  • Checks for existing entries to avoid duplicates

Implementation

# Detect host IP and configure host.docker.internal DNS mapping
configure_host_dns() {
    log_info "Detecting host IP for container networking..."
    
    local HOST_IP=""
    
    # Method 1: Try to get the primary network interface IP (not loopback)
    HOST_IP=$(ip -4 addr show 2>/dev/null | grep -oP '(?<=inet\s)\d+(\.\d+){3}' | grep -v '^127\.' | head -1)
    
    # Methods 2-4: Fallbacks...
    
    # Add host.docker.internal mapping to /etc/hosts
    if ! grep -q "host.docker.internal" /etc/hosts 2>/dev/null; then
        echo "$HOST_IP   host.docker.internal" >> /etc/hosts
        log_info "DNS mapping configured: $HOST_IP -> host.docker.internal"
    fi
}

The function integrates into the existing startup validation flow, executing after environment checks but before gateway initialization.

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