Switch base images from Docker Hub to GHCR mirror

Avoids Docker Hub 429 rate limits by pulling node:20-alpine and
nginx:stable-alpine from ghcr.io/cartsnitch/mirror/. GHCR login
now runs on all builds (not just main push) to authenticate pulls.

Ref: cartsnitch/infra#7, CAR-55

Co-Authored-By: Paperclip <noreply@paperclip.ing>

Dockerfile

@@ -1,5 +1,5 @@
 # Stage 1: Build
-FROM node:20-alpine AS build
+FROM ghcr.io/cartsnitch/mirror/node:20-alpine AS build
 
 WORKDIR /app
 
@@ -10,7 +10,7 @@ COPY . .
 RUN npm run build
 
 # Stage 2: Production
-FROM nginx:stable-alpine AS prod
+FROM ghcr.io/cartsnitch/mirror/nginx:stable-alpine AS prod
 
 COPY --from=build /app/dist /usr/share/nginx/html
 COPY nginx.conf /etc/nginx/conf.d/default.conf