I can't connect my postgresql docker container to my .NET 6 container

I have implemented an application with REST interfaces in C# on .NET 6. When the URLs are called, then the data is fetched from a Postgresql database and returned. The application is implemented as code first. That means the tables are created because of my application. This has all worked so far.

Now I am trying to put the application and the database each into a Docker container and connect them together. For this I have a Dockerfile:

#build stage
FROM mcr.microsoft.com/dotnet/sdk:6.0 as build
WORKDIR /src
COPY . .
RUN dotnet restore "./Notification/Notification.csproj" --disable-parallel
RUN dotnet publish "./Notification/Notification.csproj" -c release -o /app --no-restore

#Serve stage
From mcr.microsoft.com/dotnet/sdk:6.0
WORKDIR /app
COPY --from=build /app .
ENTRYPOINT ["dotnet", "Notification.dll"]

A docker-compose.yaml:

version: '3'

networks:
  mynetwork:
    driver: bridge

services:

   notification:
      build: .
      ports:
      - "8080:80"
      - "8081:443"
      environment:
         ASPNETCORE_URLS: "https://+;http://+"
         ASPNETCORE_HTTPS_PORT: "8001"
         ASPNETCORE_Kestrel__Certificates__Default__Password: "1234"
         ASPNETCORE_Kestrel__Certificates__Default__Path: "/https/notification.pfx"
      volumes:
         - ${USERPROFILE}.aspnethttps:/https/
      depends_on:
         - postgres
      networks:
         - mynetwork 

   postgres:
      image: postgres:9.6.21-alpine
      container_name: postgres
      ports:
      - "5432:5432"
      environment:
         - POSTGRES_USER=postgres
         - POSTGRES_PASSWORD=1234
         - POSTGRES_DB=mail
      networks:
         - mynetwork 

My AppSettings from my C# application:

{
  "Serilog": {
    "Using": [ "Serilog.Sinks.Console", "Serilog.Sinks.File" ],
    "MinimumLevel": "Debug",
    "WriteTo": [
      { "Name": "Console" },
      {
        "Name": "File",
        "Args": { "path": "../Logs/log.txt" }
      }
    ]
  },
  "AllowedHosts": "*",
  "AppSettings": {
    "ConnectionStrings": {
      "MailDb": "Server=postgres; Database = mail;  User Id = postgres; Password = 1234"
    },
    "MailServers": [
      {
        "Type": "ethereal",
        "URL": "smtp.ethereal.email",
        "Port": 587
      }
    ],
    "SlackChannels": [
      {
        "Channel": "Slackbot",
        "URL": "https://hooks.slack.com/services/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"
      }
    ]
  }
}

and my Program.cs:

using Microsoft.EntityFrameworkCore;
using Notification.Helper;
using Notification.Models;
using Notification.Service.EMail;
using NotificationService.Database;
using Serilog;

var builder = WebApplication.CreateBuilder(args);

builder.Configuration
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);

//Der ähnliche Code muss da stehen, da es ein anderes Objekt ist
var configuration = new ConfigurationBuilder()
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
    .Build();

Log.Logger = new LoggerConfiguration()
    .ReadFrom.Configuration(configuration)
    .CreateLogger();

var appSettings = new AppSettings();
builder.Configuration.GetSection("AppSettings").Bind(appSettings);

// Add services to the container.
builder.Services.Configure<AppSettings>(builder.Configuration.GetSection("AppSettings"));
builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

builder.Services.AddScoped<IMessageService, MessageService>();

builder.Services.AddDbContext<MailContext>(
    o => o.UseNpgsql(appSettings.ConnectionStrings.MailDb)
    );

var app = builder.Build();

app.UseSwagger();
app.UseSwaggerUI();

app.UseAuthorization();
app.MapControllers();

app.Run();

When I run docker-compose -f docker-compose.yaml up, both containers are set up. However, no tables are created in the postgresql container, only the database mail. When I send a GET request via Swagger, then I get the following error messages:

fail: Microsoft.EntityFrameworkCore.Database.Connection[20004] notification-notification-1 |
An error occurred using the connection to database ‘mail’ on server ”.
notification-notification-1 | fail: Microsoft.EntityFrameworkCore.Query[10100] notification-notification-1 | An exception occurred while iterating over the results of a query for context type ‘NotificationService.Database.MailContext’.

Microsoft.AspNetCore.Server.Kestrel[13] notification-notification-1 | Connection id "0HMUMN18UPCED", Request id "0HMUMN18UPCED:00000001": An unhandled exception was thrown by the application.
notification-notification-1 | System.InvalidOperationException: An exception has been raised that is likely due to a transient failure.
notification-notification-1 | —> Npgsql.NpgsqlException (0x80004005): Failed to connect to [::1]:5432