Add nginx config validation workflow

.github/nginx-check/Dockerfile.docker

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+FROM nginx:1.30.1-alpine
+
+# Validate the config set loaded by the mempool/frontend docker image.
+# Build context: repo root
+COPY nginx.conf http-basic.conf /etc/nginx/
+COPY nginx-mempool.conf /etc/nginx/conf.d/
+
+# Replay the transforms applied by docker/init.sh (with the default values
+# docker/frontend/entrypoint.sh substitutes at container start)
+RUN sed -i \
+      -e "s!127.0.0.1:80!0.0.0.0:8080!g" \
+      -e "s!user nobody;!!g" \
+      -e "s!/etc/nginx/nginx-mempool.conf!/etc/nginx/conf.d/nginx-mempool.conf!g" \
+      /etc/nginx/nginx.conf
+
+CMD ["nginx", "-t"]

.github/nginx-check/Dockerfile.production

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+FROM nginx:1.30.1-alpine
+
+# Stage the config the way production/install does: the repo is available at
+# ${NGINX_ETC_FOLDER}/mempool and production/nginx/nginx.conf becomes the main
+# config, with __NGINX_USER__ and __NGINX_ETC_FOLDER__ substituted.
+# Build context: production/
+COPY nginx /etc/nginx/mempool/production/nginx
+
+RUN cp /etc/nginx/mempool/production/nginx/nginx.conf /etc/nginx/nginx.conf && \
+    sed -i "s!__NGINX_USER__!nginx!" /etc/nginx/nginx.conf && \
+    sed -i "s!__NGINX_ETC_FOLDER__!/etc/nginx!" /etc/nginx/nginx.conf
+
+CMD ["nginx", "-t"]

.github/workflows/server-config.yml

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+name: Validate server config
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+    paths:
+      - "nginx.conf"
+      - "http-basic.conf"
+      - "nginx-mempool.conf"
+      - "production/nginx/**"
+      - ".github/nginx-check/**"
+      - ".github/workflows/server-config.yml"
+
+permissions:
+  contents: read
+
+jobs:
+  check_config:
+    runs-on: "ubuntu-latest"
+
+    name: Validate nginx config
+    steps:
+      - name: Checkout
+        uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3
+
+      - name: Validate the production nginx config
+        run: |
+          docker run --rm $(docker build -q -f .github/nginx-check/Dockerfile.production production)
+
+      - name: Validate the docker frontend nginx config
+        run: |
+          docker run --rm $(docker build -q -f .github/nginx-check/Dockerfile.docker .)