Dockerfile

@@ -68,8 +68,10 @@ FROM nginx@sha256:15e96e59aa3b0aada3a121296e3bce117721f42d88f5f64217ef4b18f458c6
 RUN addgroup -g 1000 -S netwatch && \
     adduser -u 1000 -S netwatch -G netwatch
 
+# At start-up the nginx image renders every template here into
+# conf.d; bin/entrypoint.sh says how.
 RUN rm /etc/nginx/conf.d/default.conf
-COPY nginx.conf /etc/nginx/conf.d/netwatch.conf
+COPY nginx.conf /etc/nginx/templates/netwatch.conf.template
 COPY --from=frontend /app/dist /usr/share/nginx/html
 COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
 COPY bin/entrypoint.sh /usr/local/bin/entrypoint.sh
@@ -78,6 +80,7 @@ ENV DATA_DIR=/data/reports
 RUN mkdir -p /data/reports && chown -R netwatch:netwatch /data
 VOLUME /data
 
+# The default public port; PORT changes it.
 EXPOSE 8080
 
 # The nginx image stops its container with SIGQUIT; the entrypoint

TODO.md

@@ -23,6 +23,12 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx listens on `PORT` (issue #26), 8080 when unset or empty: the
+  nginx image renders `nginx.conf` as a template at container start, filling in
+  `PORT` and no other variable. `bin/entrypoint.sh` refuses to start when `PORT`
+  is not digits only. `server_tokens off` keeps the nginx version out of
+  responses. `script/frontend-viewport-test` renders the template the same way.
+  Gzip and a `50x.html` error page are not added
 - 2026-09-29: bounded the report endpoint (issue #20): `POST /api/v1/reports`
   still needs no credentials, but each client address, as resolved through
   `TRUSTED_PROXIES`, may send `REPORTS_PER_MINUTE` (default 60) reports a

bin/entrypoint.sh

@@ -8,6 +8,18 @@
 # No set -e: kill and wait return non-zero here in normal operation.
 set -u
 
+# PORT is the public port nginx listens on, 8080 when unset or empty.
+# nginx would take a value such as localhost or unix:/tmp/x.sock as an
+# address and start anyway, so anything but digits stops the container
+# here, before either process starts.
+export PORT="${PORT:-8080}"
+case "$PORT" in
+    *[!0-9]*)
+        echo "entrypoint: PORT must be a port number, not '$PORT'" >&2
+        exit 1
+        ;;
+esac
+
 # A stop signal is only noted here; the loop below acts on it.
 stop_requested=""
 trap 'stop_requested=yes' TERM INT
@@ -23,7 +35,12 @@ backend=$!
 
 # nginx starts through the nginx image's own entrypoint, which applies
 # the image's start-up configuration and then replaces itself with
-# nginx.
+# nginx. Part of that start-up configuration renders nginx.conf into
+# conf.d with nginx listening on PORT. NGINX_ENVSUBST_FILTER limits
+# that rendering to PORT: a variable nginx itself uses, such as $uri,
+# would otherwise be replaced by an environment variable of the same
+# name.
+NGINX_ENVSUBST_FILTER='^PORT$' \
     /docker-entrypoint.sh nginx -g 'daemon off;' &
 nginx=$!
 

nginx.conf

@@ -1,7 +1,13 @@
+# A template: the nginx image renders it into conf.d at container start,
+# filling in PORT and nothing else. bin/entrypoint.sh sets PORT and that
+# limit.
 server {
-    listen 8080;
+    listen ${PORT};
     server_name _;
 
+    # Keep the nginx version out of the Server header and error pages.
+    server_tokens off;
+
     root /usr/share/nginx/html;
     index index.html;
 

script/frontend-viewport-test

@@ -61,10 +61,13 @@ main() {
     # host.
     docker network create --internal "$NETWORK" > /dev/null
 
+    # nginx.conf is a template: the image renders it over its own
+    # default.conf, with the same port and limit bin/entrypoint.sh uses.
     docker run -d --rm --name "$SERVER" \
         --network "$NETWORK" --network-alias netwatch \
+        -e PORT=8080 -e NGINX_ENVSUBST_FILTER='^PORT$' \
         -v "$ROOT/dist:/usr/share/nginx/html:ro" \
-        -v "$ROOT/nginx.conf:/etc/nginx/conf.d/default.conf:ro" \
+        -v "$ROOT/nginx.conf:/etc/nginx/templates/default.conf.template:ro" \
         "$SERVER_IMAGE" > /dev/null
 
     # The image's own entrypoint already exposes CDP on 9222 and passes