Folder created in Dockerfile not visible after running container - docker

Question

I am using Docker and have a docker-compose.yml and a Dockerfile. In my Dockerfile, I create a folder. When I build the container, I can see that the folder is created, but when I run the container, I can see all the files, but the folder that I created during the container build is not visible.
Both of these files are in the same location.
Here is docker-compose
version: '3'
services:
app:
build: .
volumes:
- ./:/app
command: tail -f /dev/null #command to leave the container on
Here is my Dockerfile
FROM alpine
WORKDIR /app
COPY . /app
RUN mkdir "test"
RUN ls
To build the container I use the command: docker-compose build --progress=plain --no-cache. Command RUN ls from Dockerfile prints me that there are 3 files: Dockerfile, docker-compose.yml and created in Dockerfile test directory.
When my container is running and i'm entering the docker to check which files are there i haven't directory 'test'.
I probably have 'volumes' mounted incorrectly. When I delete them, after entering the container, I see the 'test' folder. Unfortunately,
I want my files in the container and on the host to be sync.
This is a simple example. I have the same thing creating dependencies in nodejs. I think that the question written in this way will be more helpful to others.

Answer 1

When you mount the same volume in docker-compose it will mount that folder to the running image, and test folder will be overwritten by mounted folder.
This may work for you (to create folder from docker-compose file), but im not really sure in your use case:
version: '3'
services:
app:
build: .
volumes:
- ./:/app
command: mkdir /app/test && tail -f /dev/null
Based on your comment, below is an example on how i would use Dockerfile to build node packages and save node_modules back to host:
Dockerfile
FROM node:latest
COPY . /app
RUN npm install
Sh
#!/bin/bash
docker build -t my-image .
container_id=$(docker run -d my-image)
docker cp $container_id:/app/node_modules .
docker stop $container_id
docker rm $container_id
Or more simple way, on how I use to do it, is just to run the docker image and ssh into it:
docker run --rm -it -p 80:80 -v $(pwd):/home/app node:14.19-alpine
ssh into running container, perform npm commands then exit;