app.cs

public class LambdaApplication
{
    public string DoTheJob(string input)
    {
        Console.WriteLine(input);
        return input.ToUpper();
    }
}

docker-compose.yml

version: "3.8"
services:

  lambda:
    container_name: lambda
    build: .
    image: com.sciencevikinglabs.lambda
    stdin_open: true
    tty: true

Dockerfile

FROM public.ecr.aws/lambda/dotnet:5.0
#You can alternately also pull these images from DockerHub amazon/aws-lambda-dotnet:5.0

# Copy function code
COPY net5-lambda-template/bin/Debug/net5.0 ${LAMBDA_TASK_ROOT}

# Set the CMD to your handler (could also be done as a parameter override outside of the Dockerfile)
CMD [ "net5-lambda-template::net5_lambda_template.LambdaFunction::Handler" ]

handler.cs

using Amazon.Lambda.Core;
using Microsoft.Extensions.DependencyInjection;
using net5_lambda_template.Core;

[assembly:LambdaSerializer(typeof(Amazon.Lambda.Serialization.Json.JsonSerializer))]
namespace net5_lambda_template
{
    public class LambdaFunction
    {
        public string Handler(string input)
        {
            var host = new LambdaHost(null);
            var services = host.HostBuilder.Build().Services;
            var app = services.GetService<LambdaApplication>();

            return app.DoTheJob(input);
        }
    }
}

handler_start.cs

using Amazon.Lambda.Core;

[assembly:LambdaSerializer(typeof(Amazon.Lambda.Serialization.Json.JsonSerializer))]
namespace net5_lambda_template
{
    public class LambdaFunction
    {
        public string Handler(string input)
        {
            Console.WriteLine(input);
            return input.ToUpper();
        }
    }
}

host.cs

public class LambdaHost
{
    public IHostBuilder HostBuilder => GetHostBuilder();

    private IHostBuilder _hostBuilder;
    private readonly string[] _hostArguments;

    public LambdaHost(string[] args)
    {
        _hostArguments = args;
    }

    private IHostBuilder GetHostBuilder()
    {
        if (_hostBuilder != null)
            return _hostBuilder;

        _hostBuilder = Host.CreateDefaultBuilder(_hostArguments)
            .ConfigureServices(ConfigureServicesInternal);

        return _hostBuilder;

    }

    private static void ConfigureServicesInternal(IServiceCollection services)
    {
        services.AddTransient<LambdaApplication>();
    }
}

run.cmd

dotnet build .\net5-lambda-template.sln
docker-compose build
docker-compose run -p 9000:8080 lambda