refactor: remove BuildKit syntax directive and host network flags from Dockerfiles

Remove BuildKit syntax directive and --network=host flags from admin-web, backend, and gateway Dockerfiles to simplify builds and rely on default Docker networking for dependency downloads.

admin-web/Dockerfile

@@ -1,8 +1,7 @@
-## syntax=docker/dockerfile:1.4
 FROM node:22-alpine AS builder
 WORKDIR /app
 COPY package.json ./
-RUN --network=host npm install
+RUN npm install
 COPY . .
 RUN npm run build
 

backend/Dockerfile

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

deploy/gateway/Dockerfile

@@ -1,7 +1,6 @@
-## syntax=docker/dockerfile:1.4
 FROM alpine:3.21
 
-RUN --network=host set -eux; \
+RUN set -eux; \
     printf '%s\n' \
       'https://dl-cdn.alpinelinux.org/alpine/v3.21/main' \
       'https://dl-cdn.alpinelinux.org/alpine/v3.21/community' \