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.

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