nginx: listen on PORT, default 8080; server_tokens off (closes #26)

nginx.conf is now a template the nginx image renders into conf.d at
container start. bin/entrypoint.sh sets PORT to 8080 when unset or
empty, and stops with an error before starting anything when PORT is
not digits only: nginx would take a value such as localhost or
unix:/tmp/x.sock as an address and start anyway. NGINX_ENVSUBST_FILTER
limits the rendering to PORT, so $uri, $host and every other nginx
variable pass through unchanged. server_tokens off drops the version
from the Server header and error pages. script/frontend-viewport-test
renders the template the same way. EXPOSE still documents 8080; the
backend stays on 127.0.0.1:8081.

Model: opus-5-5

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;