Docker Compose: Understanding services for PHP & Composer

Hi,

I started learning about Docker and setting up PHP & Composer. I looked at some examples and saw the following example: https://github.com/aschmelyun/docker-compose-laravel/blob/main/docker-compose.yml

In docker-compose.yml, among others, there are two services: for php and for composer:

php:
    build:
      context: ./dockerfiles
      dockerfile: php.dockerfile
      args:
        - UID=${UID:-1000}
        - GID=${GID:-1000}
    ports:
      - "9000:9000"
    volumes:
      - ./src:/var/www/html:delegated
    networks:
      - laravel

  composer:
    build:
      context: ./dockerfiles
      dockerfile: php.dockerfile
      args:
        - UID=${UID:-1000}
        - GID=${GID:-1000}
    volumes:
      - ./src:/var/www/html
    depends_on:
      - php
    entrypoint: [ 'composer', '--ignore-platform-reqs' ]
    networks:
      - laravel

and both are specified to use the same php.dockerfile:

build:
    context: ./dockerfiles
    dockerfile: php.dockerfile

Finally, php.dockerfile looks like this (I left out some parts so it wouldn't be too long here, you can see the full version here https://github.com/aschmelyun/docker-compose-laravel/blob/main/dockerfiles/php.dockerfile):

FROM php:8-fpm-alpine

ARG UID
ARG GID

ENV UID=${UID}
ENV GID=${GID}

RUN mkdir -p /var/www/html

WORKDIR /var/www/html

COPY --from=composer:latest /usr/bin/composer /usr/local/bin/composer

# MacOS staff group's gid is 20, so is the dialout group in alpine linux. We're not using it, let's just remove it.
RUN delgroup dialout

RUN addgroup -g ${GID} --system laravel
RUN adduser -G laravel --system -D -s /bin/sh -u ${UID} laravel

RUN sed -i "s/user = www-data/user = laravel/g" /usr/local/etc/php-fpm.d/www.conf
RUN sed -i "s/group = www-data/group = laravel/g" /usr/local/etc/php-fpm.d/www.conf

RUN docker-php-ext-install pdo pdo_mysql
    
USER laravel

CMD ["php-fpm", "-y", "/usr/local/etc/php-fpm.conf", "-R"]

4 (interrelated) questions:

Question 1: Since in docker-compose.yml both services, php and composer, use the same php.dockerfile, does this mean the following: Only one image will be build (for php service, as it comes before composer service in docker-compose.yml file), and from that same image two separate containers will be created: one for php service and another for composer service, right?

Question 2: If the answer to Question 1 is yes (if separate containers will be created) - does that make any sense because those two containers will be identical? Also, In php.dockerfile there is:

COPY --from=composer:latest /usr/bin/composer /usr/local/bin/composer

This means that the "php" container will already have Composer in it, so I don't see the point of having a separate identical container for "composer"?

Question 3: The following is specified in docker-compose.yml for the composer service:

entrypoint: [ 'composer', '--ignore-platform-reqs' ]

If I understand correctly, this means that every time the “composer” container is started, the composer --ignore-platform-reqs will be executed, right? But that doesn’t make sense because composer --ignore-platform-reqs will do nothing as there is no any command specified there: composer COMMAND --ignore-platform-reqs (for example, composer install –ignore-platform-reqs).

Question 4: Finally, the last question is about the php.dockerfile, there are a few things that I don't think are needed and don't do anything, but maybe I'm wrong (please correct me if I’m wrong):

• WORKDIR /var/www/html - I don't see the purpose of this in that particular php.dockerfile;
• RUN delgroup dialout - same goes for this; even if we keep that dialout group, I don't see how it can create any problem?
• USER laravel - this is applied at the very end, so I also don't see the purpose of this (without that it should work the same, without any problems)?