feat: enable host network access during Docker build stages for dependency downloads

Add BuildKit syntax directive and --network=host flag to npm install, go mod operations, and apk package installation to allow direct host network access during builds, bypassing Docker's default bridge network for improved reliability and performance of dependency downloads.

backend/Dockerfile

@@ -1,10 +1,11 @@
+## syntax=docker/dockerfile:1.4
 FROM golang:1.23-alpine AS builder
 WORKDIR /src
 COPY go.mod ./
 COPY go.sum* ./
 COPY . .
-RUN go mod tidy
-RUN go mod download
+RUN --network=host go mod tidy
+RUN --network=host go mod download
 RUN CGO_ENABLED=0 GOOS=linux go build -o /out/nexavpn-api ./cmd/api
 
 FROM alpine:3.21