fix: route all Docker builds through local Nexus mirrors

- All Node Dockerfiles rewritten with NODE_IMAGE + NPM_REGISTRY build args
  defaulting to local Nexus proxies (171.22.25.73:5000/library/node:20-alpine
  and http://mirror:8081/repository/npm-group/)
- Add extra_hosts: mirror:host-gateway to every build section so the
  mirror hostname resolves during docker build
- Replace nuget.org with nuget.docker.config (Nexus mirror) in api/admin-api
  Dockerfiles to fix NuGet restore in Iranian network
- Rewrite admin-web and website Dockerfiles (were referencing non-existent
  meezi-node:20-alpine base image with no npm install step)
- Update dotnet image defaults to 171.22.25.73:5002 MCR proxy in admin-api
  and docker-compose.admin.yml

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/web/Dockerfile

@@ -1,10 +1,12 @@
-FROM node:20-alpine AS deps
+ARG NODE_IMAGE=node:20-alpine
+
+FROM ${NODE_IMAGE} AS deps
 WORKDIR /app
 COPY web/dashboard/package*.json ./
-RUN npm config set registry https://registry.npmmirror.com \
-    && npm install --legacy-peer-deps --ignore-scripts
+ARG NPM_REGISTRY=https://registry.npmjs.org
+RUN npm install --legacy-peer-deps --ignore-scripts --registry ${NPM_REGISTRY}
 
-FROM node:20-alpine AS builder
+FROM ${NODE_IMAGE} AS builder
 WORKDIR /app
 
 ARG NEXT_PUBLIC_API_URL=http://localhost:5080
@@ -15,7 +17,7 @@ COPY --from=deps /app/node_modules ./node_modules
 COPY web/dashboard/ .
 RUN npm run build
 
-FROM node:20-alpine AS runner
+FROM ${NODE_IMAGE} AS runner
 WORKDIR /app
 
 ENV NODE_ENV=production
@@ -32,5 +34,4 @@ COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
 
 USER nextjs
 EXPOSE 3000
-
 CMD ["node", "server.js"]