Docker-Compose Overwriting Dockerfile Data

How to keep docker-compose volumes from overwriting container data.

This little challenge took me longer than I had hoped to figure out, so I figured I would blog about it in case I run into the problem again.

Here was the problem. I dockerized a small coding assignment project. It has a backend server in python/bottle.py and frontend running Aurelia. This is what my frontend dockerfile looked like:

1# use latest version of nodejs
2  FROM node:lts-alpine
3
4  # install aurelia-cli to build the app & http-server to serve static contents
5  RUN npm i -g aurelia-cli@0.16.0
6
7  # set working directory to app
8  # henceforth all commands will run inside this folder
9  WORKDIR /app
10
11  # copy package.json related files first and install all required dependencies
12  COPY ./frontend/package*.json ./
13  RUN npm install

This was my docker-compose file:

1version: '3.9'
2
3services:
4  backend:
5    build:
6      context: .
7      dockerfile: Dockerfile.backend
8    command: python api.py
9    volumes:
10      - ./backend:/backend
11    ports:
12      - '8085:8085'
13  frontend:
14    build:
15      context: .
16      dockerfile: Dockerfile.frontend
17    command: au run --watch
18    volumes:
19      - ./frontend:/app
20    ports:
21      - '3001:3001'
22      - '9000:9000'

The Problem

The dockerfile was installing all the front-end requirements in /app/node_modules. Docker compose was mounting my local ./frontend directory to my container's app directory. For me it was working fine because I had a node_modules directory in my ./frontend directory from running npm install locally. When someone else would check out the project and run docker-compose up they were getting errors because they were missing modules. Their host ./frontend directory did not contain node_modules so it wasn't work.

The Fix

The easiest fix I found was to add the add another volume which essentially keeps the node_modules directory on the server. It is a one-line fix in my docker compose file.

1frontend:
2  build:
3    context: .
4    dockerfile: Dockerfile.frontend
5  command: au run --watch
6  volumes:
7    - ./frontend:/app
8    - /app/node_modules
9  ports:
10    - '3001:3001'
11    - '9000:9000'