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.

nginx/Dockerfile

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