docker-compose build and docker build giving different results

I have created a simple Dockerfile to install apache with PHP and then install packages from composer.json.

FROM php:7-apache

WORKDIR /var/www/html

COPY ./src/html/ .
COPY composer.json .

RUN apt-get update
RUN apt-get install -y unzip

# Install Composer
RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer

RUN composer update

When I run docker build -t my-web-server . followed by docker run -p 8080:80 my-web-server, everything works fine and the packages install.

But when I use a docker-compose file:

version: "3.9"
services:
  ecp:
    build: .
    ports:
      - "8080:80"
    volumes:
      - ./src:/var/www

and perform docker-compose build followed by docker-compose up The packages do not install and I just index.php is taken across to the container

my current file structure:

src
 |-- html
     |-- index.php
composer.json
docker-compose.yaml
Dockerfile

When docker-compose is building the image all the console outputs are identical to that of docker build

Answer

Your two approaches are not identical. You are using volumes in your docker compose, and not in your docker call. Your problem lies there.

More specifically, notice that in your docker compose you are mounting your host’s ./src to your container’s ./var/www – which is not the giving you the correct structure, since you “shadow” the container’s folder that contains your composer.json (which was copied to the container at build time).

To avoid such confusion, I suggest that if you want to mount a volume with your compose (which is a good idea for development), then your docker-compose.yml file should mount the exact same volumes as the COPY commands in your Dockerfile. For example:

volumes:
  - ./src/html:/var/www/html
  - ./composer.json:/var/www/html/composer.json

Alternatively, remove the volumes directive from your docker-compose.yml.

Note that it can be a cause for additional problems and confusion to have a file (in your case composer.json) copied to a folder in the container, while having the same folder also copied to the container as is. It is best to have the structure on the container mimic the one on the host as closely as possible.