Docker volume clears folder in container on windows 10 - docker

Question

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 in the host:
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.
In addition, the command: docker volume ls shows an empty list.
I'm running docker 18.09.3 on windows 10.
Is there anything wrong with the configuration? How is it supposed to work?

Answer 1

Adding the volume to your service will remove all the files /usr/src/app and mount the content of ./app from your host machine. This also means that all files generated by running yarn in the docker image will be lost because they exist only in the docker image. This is the expected behaviour of adding volume in docker and it is not a bug.
volumes:
- ./app:/usr/src/app
Usually and for none development Envs you don't need the volume at all here.
If you would like to see the files on your host, you need to run yarn command from docker-compose (you can use an entry point)