Docker compose with Laravel question

I recently started learning Docker and I got the basic concepts. I am now trying to build an image for my Laravel project. I would appreciate some help, since I don't really understand some things. My Dockerfile

FROM php:7.4-fpm

ARG user
ARG uid

RUN apt-get update && apt-get install -y \
    git \
    curl \
    libpng-dev \
    libonig-dev \
    libxml2-dev \
    zip \
    unzip

RUN apt-get clean && rm -rf /var/lib/apt/lists/*

RUN docker-php-ext-install pdo_mysql mbstring exif pcntl bcmath gd

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

RUN useradd -G www-data,root -u $uid -d /home/$user $user
RUN mkdir -p /home/$user/.composer && \
    chown -R $user:$user /home/$user

WORKDIR /var/www

RUN npm install tailwindcss

USER $user

And my docker-compose.yml

version: '3'

services:
  app:
    container_name: expenses_app
    build:
      args:
        user: expenses_user
        uid: 1000
      context: ./
      dockerfile: Dockerfile
    image: expenses-app
    restart: unless-stopped
    working_dir: /var/www
    volumes:
      - ./:/var/www
    networks:
      - expenses_network
      - db_network
    depends_on:
      - db
      - nodejs
  db:
    image: mysql:5.7
    container_name: expenses_db
    restart: unless-stopped
    ports:
      - "3306:3306"
    environment:
      MYSQL_DATABASE: ${DB_DATABASE}
      MYSQL_ROOT_PASSWORD: ${DB_PASSWORD}
      MYSQL_PASSWORD: ${DB_PASSWORD}
      MYSQL_USER: ${DB_USERNAME}
      SERVICE_TAGS: dev
      SERVICE_NAME: mysql
    volumes:
      - mysql-volume:/var/lib/mysql
    networks:
      - db_network
  nginx:
    image: nginx:alpine
    container_name: expenses_nginx
    restart: unless-stopped
    ports:
      - 8000:80
    volumes:
      - ./:/var/www
      - ./config/docker-compose/nginx:/etc/nginx/conf.d
    networks:
      - expenses_network
  nodejs:
    image: node:14-alpine
    container_name: expenses_nodejs
    restart: unless-stopped
    volumes:
     - ./:/var/www
    networks:
      - expenses_network

volumes:
  mysql-volume:

networks:
  db_network:
  expenses_network:
    driver: bridge

My idea is to add all needed commands that setup the project in the Dockerfile (composer install, npm install, php artisan migrate etc.), but for whatever reason if I add composer install I get - composer.json not found. If I hae an npm install command (in my case tailwind), I get - npm is not installed.

I don't really understand what should I put in the docker-compose and what in the Dockerfile. My understanding is that in the compose file I need to describe what services I want to use and in the Dockerfile I can execute commands (simplifying it of course). But if that's the case, why don't I have composer.json since I'm mounting my project dir? Why don't I have npm since I am adding the node:14-alpine image in the compose file?

Sorry for the wall of text and thanks to whoever reads this.