Differences in code between local project and Dockerized project break the app

I'm trying to dockerize my current pet project in which I use a NodeJS (ExpressJS) as a backend, React as a frontend and PostgreSQL as a database. On both backend and frontend I use TypeScript instead of JavaScript. I'm also using a Prisma as ORM for my database. I decided to have a standard three container's architecture, one for backend, one for database and one for frontend app. My Dockerfile's are as follows:

Frontend's Dockerfile

FROM node:alpine

WORKDIR /usr/src/frontend

COPY package*.json ./

RUN npm install

COPY . .
EXPOSE 3000

CMD ["npm", "run", "start"]

Backend's Dockerfile

FROM node:lts

WORKDIR /usr/src/backend

COPY package*.json ./

RUN npm install

COPY . .
EXPOSE 8000

RUN npx prisma generate
CMD ["npm", "run", "dev"]

there's also a .dockerignore file in the backend folder:

node_modules/

and my docker-compose.yml looks like this:

version: '3.9'

services:
  db:
    image: 'postgres'
    ports:
      - '5432:5432'
    environment:
      POSTGRES_USER: 'postgres'
      POSTGRES_PASSWORD: 'postgres'
      POSTGRES_DB: 'hucuplant'
  server:
    build:
      context: ./backend_express 
    ports: 
      - "8000:8000"
    environment:
      DATABASE_URL: 'postgresql://postgres:postgres@localhost:5432/hucuplant?schema=public'
  client:
    build:
      context: ./frontend
    ports:
      - "3000:3000"

After doing a docker-compose up --build everything starts well but when I try to register a new user on my site then I get the following error:

Error: 
hucuplant-server-1  | Invalid `prisma.user.findUnique()` invocation in
hucuplant-server-1  | /usr/src/backend/src/routes/Auth.ts:44:57
hucuplant-server-1  | 
hucuplant-server-1  |   41 auth.post("/register", async (req: Request, res: Response) => {
hucuplant-server-1  |   42   const { email, username, password } = req.body;
hucuplant-server-1  |   43 
hucuplant-server-1  | → 44   const usernameResult: User | null = await prisma.user.findUnique({
hucuplant-server-1  |          where: {
hucuplant-server-1  |        ?   username?: String,
hucuplant-server-1  |        ?   id?: Int,
hucuplant-server-1  |        ?   email?: String
hucuplant-server-1  |          }
hucuplant-server-1  |        })

However, the existing code in my Auth.ts file on the line 44 looks like this:

auth.post("/register", async (req: Request, res: Response) => {
  const { email, username, password } = req.body;

  const usernameResult: User | null  = await prisma.user.findUnique({
    where: {
      username: username,
    },
  });

When I run my project locally everything works just fine but when I try to run the containerized app then those things break and differ quite much. What is causing that? How do I fix that?