Why can docker not find copied files?

I've tried a bunch of different solutions to similar/identical questions on here and other places and it still won't work. My goal is to run multiple docker containers for my backend and frontend (and a few others but i cut them out here since they start and run fine). The backend and frontend containers won't start up since docker cannot seem to be able to find the manage.py (for running the backend) and package.json (for installing the frontend dependencies) files.

My folder structure:

- docker-compose.yml
- frontend/
  - Dockerfile
  - package.json
  - node project files...
- backend/
  - Dockerfile
  - manage.py
  - more django project files...

My docker-compose.yml file:

version: '3.4'

services:
  backend:
    build:
      context: ./backend
      dockerfile: Dockerfile
    container_name: 'backend'
    command: python manage.py runserver 0.0.0.0:8080
    volumes:
      - ./data/backend:/backend_app
    ports:
      - '8080:8080'
    restart: always

  frontend:
    image: node:18.14.2
    build:
      context: ./frontend
      dockerfile: Dockerfile
    container_name: 'frontend'
    command: npm run preview
    ports:
      - '3000:3000'
    volumes:
      - ./data/frontend:/frontend_app

frontend/Dockerfile:

FROM node:18.14.2

EXPOSE 3000

WORKDIR /frontend_app

COPY package*.json ./

RUN npm install

COPY . .

CMD ["npm", "run", "preview"]

backend/Dockerfile:

FROM python:3.10-slim

EXPOSE 8080

ENV PYTHONDONTWRITEBYTECODE=1

ENV PYTHONUNBUFFERED=1

WORKDIR /backend_app

COPY requirements.txt .

RUN python -m pip install --upgrade pip
RUN python -m pip install -r requirements.txt

COPY . .

CMD ["python", "manage.py", "runserver", "0.0.0.0:8080"]

My (partial) docker console output when running docker compose up:

frontend          | npm
frontend          |  ERR! code ENOENT
frontend          | npm ERR! syscall open
frontend          | npm ERR! path /frontend_app/package.json
frontend          | npm
frontend          |  ERR! errno -2
frontend          | npm ERR! enoent ENOENT: no such file or directory, open '/frontend_app/package.json'
frontend          | npm ERR! enoent This is related to npm not being able to find a file.
frontend          | npm ERR! enoent 
frontend          | 
frontend          | 
frontend          | npm ERR! A complete log of this run can be found in:
frontend          | npm ERR!     /root/.npm/_logs/2023-07-17T22_35_21_039Z-debug-0.log
backend           | python: can't open file '/backend_app/manage.py': [Errno 2] No such file or directory
backend exited with code 2

You are mounting ./data/{frontend,backend}, overwriting anything you did on build. Your directory structure does not suggest that this is the right path.

You should mount ./{frontend,backend} instead.

Additionally, due to fetching the dependencies (node_modules) on build, they will not be available when you do docker compose up, since you overwrite the container file system with the mounted host directory. So you need to ensure that you install the dependencies such that they will exist on the host system. All in all, mixing the build with the volume mounts seems a bit strange.