Docker volume clears folder in container

I’ve created a simple docker with a nodejs server.

FROM node:12.16.1-alpine
WORKDIR /usr/src

COPY ./app/package.json .
RUN yarn

COPY ./app ./app

This works great and the service is running.

Now I’m trying to run the docker with a volume for local development using docker compose:

version: "3.4"
services:
    web:
        image: my-node-app
        volumes:
            - ./app:/usr/src/app
        ports:
            - "8080:8080"
        command: ["yarn", "start"]        
        build:
            context: .
            dockerfile: ./app/Dockerfile

This is my folder structure:
enter image description here

The service works without the volume. When I add the volume, the /usr/src/app is empty (even though it is full as shown in the folder structure).

Inspecting the docker container I get the following mount config:

"Mounts": [
        {
            "Type": "bind",
            "Source": "/d/development/dockerNCo/app",
            "Destination": "/usr/src/app",
            "Mode": "rw",
            "RW": true,
            "Propagation": "rprivate"
        }
    ],

But still, browsing to the folder via shell of vscode show it as empty.

I’m running docker 18.09.3 on windows 10.

Is there anything wrong with the configuration? How is it supposed to work?

Source: StackOverflow