Oh, for upstream/base? If a Docker Hub image was set up with automated builds, it would automatically rebuild the image once its base was rebuilt. It looks like nginx was set up this way, but nginx-proxy was not. So you have to rebuild nginx-proxy manually.
~$ git clone https://github.com/jwilder/nginx-proxy
~$ cd nginx-proxy
~$ docker build -f Dockerfile.alpine -t my-own-tag-name/nginx-proxy:alpine .
~$ cd ..
~$ sed -i -e 's/image: jwilder\/nginx-proxy.*/image: my-own-tag-name\/nginx-proxy:alpine/' docker-compose.yaml
Alternately, you could fork the nginx-proxy repo, build it, push it to your own Docker Hub account, and add the Repository Links[1] back to nginx. Then you could use TravisCI to automatically pull in and rebuild changes from the parent nginx-proxy. This way you get automated builds when base is updated, and when nginx-proxy is updated.
Yes, it is entirely possible to do the right thing.
The trouble is 9 times out of 10, the only place you find it documented is in some random forum post where cargo-cult silliness gets critiqued, rather than by the folks publishing these how-tos.
[1] https://docs.docker.com/docker-hub/builds/#repository-links