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