Serving new static files via nginx stored in php-fpm container after update

Simply do not use a volume for this.

You should treat docker images as "monolithic packages" that contain your dependencies (nginx) and your app's files (images, js, css...). There's no need to treat your app's files any differently than nginx itself, it's all part of the single docker image.

Without a volume, you run v1 of your image, nginx sees the v1 files. You run v2 of your image, nginx sees the v2 files.

Volumes are intended to be used when you actually want to keep files between container versions (such as databases, file uploads...). Not for your site's static assets.

My intention is to use nginx container for multiple PHP apps and I need to refrain from killing it whenever I update one of the apps being served. Is this a bad decision?

Yes, this is bad design. If you want to run multiple apps, you should run 1 Docker container per app. That way, when you release a new version of one app, you only need to restart that container. Containers aren't supposed to be treated like traditional virtual machines where you "SSH into" and manually configure things. Containers are "throw-away". New version of the app? just replace the container with a new one with a newer image.

First option: don't use a volume. If you want to have the files accessible from the image build, and don't need persistence, then the volume isn't helping with your workflow.

Second option: delete the previous volume between runs and use a named volume, which docker will initialize with the image contents.

Third option: modify the image build and container entrypoint to save the directory off to a different location during the build, and restore that location into the volume on container startup in the entrypoint. I've got an implementation of this in the save-volume and load-volume scripts in my base image. It gets more complicated when you want to merge the contents of the volume with the contents of the host, and you'll need to decide how to handle files getting deleted and what changes to save from the previous runs.