There are multiple parts of Compose that deal with environment variables in one sense or another. So how do I pass Environment variables in Compose ( docker-compose )

According to the documentation If you have multiple environment variables, you can substitute them by adding them to a default environment variable file named .env or by providing a path to your environment variables file using the –env-file command line option.

version: '3.9'
services:
  nginx:
    image: nginx:stable-alpine
    container_name: nginx
    ports:
      - 80:80
      - 443:443
    restart: always
    volumes:
        - ./nginx/nginx.conf:/etc/nginx/nginx.conf
  postgres:
    container_name: postgres
    image: postgres:13-alpine
    environment:
      - POSTGRES_USER=${DB_USER}
      - POSTGRES_PASSWORD=${DB_PASSWORD}
      - POSTGRES_DB=${DB_DATABASE}
    volumes:
      - ./pgdata:/var/lib/postgresql/data
      - ./database/app.sql:/docker-entrypoint-initdb.d/app.sql
    restart: always
    ports:
      - "35000:5432"
    networks:
      - app_network

  app-api:
    container_name: app-api
    build:
      dockerfile: Dockerfile
      context: ./app-api
      target: production
    environment:
      - DB_TYPE=${DATABASE_TYPE}
      - POSTGRES_HOST=${DB_HOST}
      - POSTGRES_USER=${DB_USER}
      - POSTGRES_PASS=${DB_PASSWORD}
      - POSTGRES_DB=${DB_DATABASE}
      - POSTGRES_PORT=${DB_PORT}
      - APP_PORT=${SERVER_PORT}
      - NODE_ENV:production

      ## AWS
      - AWS_S3_ACCESS_KEY=${AWS_S3_ACCESS_KEY}
      - AWS_S3_SECRET_ACCESS_KEY=${AWS_S3_SECRET_ACCESS_KEY}
      - AWS_S3_BUCKET=${AWS_S3_BUCKET}
      - AWS_S3_REGION=${AWS_S3_REGION}
    ports:
      - "5050:80"
    volumes: 
      - ./pgadmin-data:/var/lib/pgadmin
    depends_on: 
      - postgres
    links:
      - postgres
    networks:
      - app_network

  pgadmin:
    container_name: pgadmin
    image: dpage/pgadmin4
    restart: always
    environment:
      - PGADMIN_DEFAULT_EMAIL=${PGADMIN_DEFAULT_EMAIL}
      - PGADMIN_DEFAULT_PASSWORD=${PGADMIN_DEFAULT_PASSWORD}
      - PGADMIN_LISTEN_PORT=${PGADMIN_LISTEN_PORT}
    restart: always
    ports:
      - "5400:5400"
    depends_on:
      - postgres
    links:
      - postgres
    networks:
      - app_network

>Solution :

you can use it like this. you have to pass value in each environment variable.

server:
    environment:
      - AWS_S3_ACCESS_KEY=ABCJQHWEQJHWQ
      - AWS_S3_SECRET_ACCESS_KEY=ASKJHDAKJHNAWKLHEN
      - AWS_S3_BUCKET=abc-text
      - AWS_S3_REGION=eu-west-1
    ports:
      - "5050:80"
    volumes: 
      - ./pgadmin-data:/var/lib/pgadmin
    depends_on: 
      - postgres
    links:
      - postgres
    networks:
      - app_network