System.ArgumentException: Host can't be null while creating db in Docker

I'm trying to create a .Net and Postgresql's docker container, but it throws the error when I started to build:

8.578 System.ArgumentException: Host can't be null

This is how I settled the Dockerfile:

FROM mcr.microsoft.com/dotnet/aspnet:7.0 AS base
WORKDIR /app
EXPOSE 80
EXPOSE 443

FROM mcr.microsoft.com/dotnet/sdk:7.0 AS build
WORKDIR /src
COPY ["file.csproj", "./"]
RUN dotnet restore "./file.csproj"
COPY . .
RUN dotnet tool install --global dotnet-ef --version 7.0.3
RUN /root/.dotnet/tools/dotnet-ef migrations add InitialCreate --project EnrollApp.csproj -v
RUN /root/.dotnet/tools/dotnet-ef database update

WORKDIR "/src/."
RUN dotnet build "file.csproj" -c Release -o /app/build
FROM build AS publish
RUN dotnet publish "file.csproj" -c Release -o /app/publish

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", ".dll"]

This is the docker-compose file

version: '3.4'

networks:
  dev:
    driver: bridge
services:
 db:
   image: postgres
   environment:
     POSTGRES_USER: postgres
     POSTGRES_DB: my_db
     POSTGRES_PASSWORD: example
   ports:
      - 5433:5432
   restart: always
   networks:
      - dev
   volumes:
     - postgres-data:/var/lib/postgresql/data
 adminer:
   image: adminer
   restart: always
   ports:
     - 8080:8080
   networks:
     - dev
 app:
   image: docker.io/library/appdemo
   build:
     context: .
     dockerfile: ./Dockerfile
   environment:
      - ConnectionStrings__DefaultConnection= User ID =postgres;Password=example;Server=db;Port=5432;Database=my_db; Integrated Security=true;Pooling=true;
      - ASPNETCORE_URLS=http://+:80
   ports:
     - 5000:80
   depends_on:
     - db
   networks:
      - dev
volumes:
 postgres-data:

And this is the ConnectionString configuration in Program.cs:

builder.Services.AddDbContext<DbContext>(o => {
     o.UseNpgsql(builder.Configuration.GetConnectionString("DefaultConnection"));
    }
);

And finally, this is the connectionString in AppSettings.json

"ConnectionStrings": {
      "DefaultConnection": "Server=db;Port=5432;Database=my_db;Username=postgres;Password=example"
    }