fix(nginx): build nginx image instead of bind-mounting config

Coolify's build context and runtime mount paths don't share the repo
tree the way a plain docker compose up does — the bind mount of
./nginx/default.conf failed at container start because the host path
didn't exist. Switch to a build: context: ./nginx pattern (same as
the signup service) so the config is baked into the image at build
time.

docker-compose.yml

@@ -17,7 +17,8 @@ services:
   # happens behind a TLS-terminating proxy — so SameSite=None cookies would be
   # dropped by browsers on cross-origin requests. See nginx/default.conf.
   nginx:
-    image: nginx:alpine
+    build:
+      context: ./nginx
     restart: unless-stopped
     depends_on:
       - couchdb
@@ -25,8 +26,6 @@ services:
       SERVICE_FQDN_NGINX_5984:
     expose:
       - "5984"
-    volumes:
-      - ./nginx/default.conf:/etc/nginx/conf.d/default.conf:ro
     networks:
       - default
       - coolify

nginx/Dockerfile

@@ -0,0 +1,3 @@
+FROM nginx:alpine
+COPY default.conf /etc/nginx/conf.d/default.conf
+EXPOSE 5984