fix: clean up nginx read-only filesystem approach (#125)

Remove Dockerfile /tmp workaround hacks (NGINX_ENVSUBST_OUTPUT_DIR and sed).
Use tmpfs with uid=101,gid=101 in docker-compose.yml instead, so the
nginx user can write to /etc/nginx/conf.d directly under read_only: true.

docker-compose.yml

@@ -52,7 +52,7 @@ services:
       - /tmp:noexec,nosuid,size=64m
       - /var/cache/nginx:noexec,nosuid,size=64m
       - /var/run:noexec,nosuid,size=64m
-      - /etc/nginx/conf.d:noexec,nosuid,size=1m
+      - /etc/nginx/conf.d:noexec,nosuid,size=1m,uid=101,gid=101
     cap_drop:
       - ALL