Unable to connect to PostgreSQL in Docker from another Docker container

I have 2 docker containers as shown below. I can connect to PostgresSQL in docker by calling Test.Main() method without exception through a net6.0 C# Console App. But when I try to call Test.Main() from ASP.NET docker container I am unable to connect to PostgresSQL and get the exception Failed to connect to 127.0.0.1:5432:

{Npgsql.NpgsqlException (0x80004005): Failed to connect to 127.0.0.1:5432
 ---> System.Net.Sockets.SocketException (111): Connection refused
   at Npgsql.Internal.NpgsqlConnector.Connect(NpgsqlTimeout timeout)
   at Npgsql.Internal.NpgsqlConnector.Connect(NpgsqlTimeout timeout)
   at Npgsql.Internal.NpgsqlConnector.RawOpen(SslMode sslMode, NpgsqlTimeout timeout, Boolean async, CancellationToken cancellationToken, Boolean isFirstAttempt)
   at Npgsql.Internal.NpgsqlConnector.<Open>g__OpenCore|215_1(NpgsqlConnector conn, SslMode sslMode, NpgsqlTimeout timeout, Boolean async, CancellationToken cancellationToken, Boolean isFirstAttempt)
   at Npgsql.Internal.NpgsqlConnector.Open(NpgsqlTimeout timeout, Boolean async, CancellationToken cancellationToken)
   at Npgsql.PoolingDataSource.OpenNewConnector(NpgsqlConnection conn, NpgsqlTimeout timeout, Boolean async, CancellationToken cancellationToken)
   at Npgsql.PoolingDataSource.<Get>g__RentAsync|28_0(NpgsqlConnection conn, NpgsqlTimeout timeout, Boolean async, CancellationToken cancellationToken)
   at Npgsql.NpgsqlConnection.<Open>g__OpenAsync|45_0(Boolean async, CancellationToken cancellationToken)
   at Npgsql.NpgsqlConnection.Open()
   at APIDA.DBModels.Test.Main() in Version.cs:line 34}

C:UsersHT>docker ps
CONTAINER ID   IMAGE        COMMAND                  CREATED          STATUS          PORTS                                           NAMES
bdc65451aeda   gb3api:dev   "tail -f /dev/null"      17 minutes ago   Up 16 minutes   0.0.0.0:49160->80/tcp, 0.0.0.0:49159->443/tcp   GB3API
95b6a289f4a8   postgres     "docker-entrypoint.s…"   47 minutes ago   Up 47 minutes   0.0.0.0:5432->5432/tcp                          gb-postgres

ASP.NET DockerFile is

FROM mcr.microsoft.com/dotnet/aspnet:6.0 AS base
WORKDIR /app
EXPOSE 80
EXPOSE 443

FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
WORKDIR /src
COPY ["GB3API/GB3API.csproj", "GB3API/"]
RUN dotnet restore "GB3API/GB3API.csproj"
COPY . .
WORKDIR "/src/GB3API"
RUN dotnet build "GB3API.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "GB3API.csproj" -c Release -o /app/publish /p:UseAppHost=false

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "GB3API.dll"]

Test.Main

public class Test
{
    public static void Main()
    {
        string connectionString = "Host=127.0.0.1;Database=postgres;Username=postgres;Password=xxxxx";

        // Create a new PostgreSQL connection
        using (NpgsqlConnection connection = new NpgsqlConnection(connectionString))
        {
            try
            {
                // Open the connection
                connection.Open();

                // Check if the connection is open
                if (connection.State == System.Data.ConnectionState.Open)
                {
                    Console.WriteLine("Connected to PostgreSQL!");

                    // Perform your database operations here

                    // Close the connection when you're done
                    connection.Close();
                    Console.WriteLine("Connection closed.");
                }
                else
                {
                    Console.WriteLine("Failed to open the connection.");
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error: " + ex.Message);
            }
        }

        Console.ReadLine();
    }
}