NestJS .env not connecting to Docker Container DB

I'm working on a nestjs project currently and I'm trying to connect it to a mysql/mariadb database with typoeorm right now which is running in a docker container (as well as the project itself).

Following the documentation of nestjs "configuration" I installed the package via the cli and imported it into the app.module.ts

import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';
import { TypeOrmModule } from '@nestjs/typeorm';
import { AppController } from './app.controller';
import { AppService } from './app.service';
import { AuthModule } from './auth/auth.module';
import { User } from './auth/user.entity';
import { MoviesModule } from './movies/movies.module';

@Module({
  imports: [
    ConfigModule.forRoot({ //<- imported the config module
      isGlobal: true,
    }),
    TypeOrmModule.forRoot({
      type: 'mariadb',
      host: process.env.DB_HOST, //<- now trying to use the .env files data 
      port: parseInt(process.env.DB_PORT),
      username: process.env.DB_USER,
      password: process.env.DB_PASS,
      database: process.env.DB_NAME,
      entities: [User],
      synchronize: true,
    }),
    AuthModule,
    MoviesModule],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

My .env looks like this

DB_USER=root
DB_PASS=example
DB_HOST=mariadb
DB_NAME=lizenzdb
DB_PORT=3306

I spun up the docker containers via docker-compose, my docker-compose looks like this:

version: '3.5'

services:
  etk-be:
    build: ./backend/
    image: etk-lizenzdb-backend:final
    container_name: etk-be
    restart: unless-stopped
    ports:
      - 3000:3000
    networks: 
      - etk

  mariadb:
    image: mariadb
    container_name: mariadb
    command: --default-authentication-plugin=mysql_native_password
    restart: unless-stopped
    environment:
      MYSQL_ROOT_PASSWORD: example
      MYSQL_ALLOW_EMPTY_PASSWORD: 'no'
      MYSQL_DATABASE: lizenzdb
    ports:
      - 3306:3306
    volumes:
      - ./data/db:/var/lib/mysql
    networks:
      - etk

  adminer:
    image: adminer
    container_name: adminer
    restart: unless-stopped
    ports:
      - 8080:8080
    networks:
      - etk

  phpmyadmin:
    image: phpmyadmin/phpmyadmin
    container_name: phpmyadmin
    restart: unless-stopped
    ports:
      - 8081:80
    environment:
      - PMA_HOST=mariadb
      - PMA_USER=root
      - PMA_PASSWORD=example
    networks:
      - etk

networks:
    etk:
        driver: bridge

I tried connecting the project and database outside the docker containers already and it worked. All I have to change for the docker container is the db host from localhost to the name of the docker container itself (mariadb), but it won't connect to the database.

You have the .env file, however, you need to let docker know to use it. I am adding an example of it to show how you can use it.

services:
  etk-be:
    build: ./backend/
    image: etk-lizenzdb-backend:final
    container_name: etk-be
    restart: unless-stopped
    ports:
      - 3000:3000
    networks: 
      - etk
    env_file: 
      - /path/to/envfile

  mariadb:
    ..

env file documentation