Providing a connection string in EF core

appsettings.json

{
    "ConnectionStrings": {
        "MyDb": "Data Source=127.0.0.1;Initial Catalog=mydb;persist security info=True;user id=sa;password=*****"
    }
}

MyDbContext.cs

public class MyDbContext : MyContext
    {
        private readonly string connectionString;
        public MyDbContext(string connectionString) : base()
        {
            this.connectionString = connectionString;
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
            {
#warning To protect potentially sensitive information in your connection string, you should move it out of source code. See http://go.microsoft.com/fwlink/?LinkId=723263 for guidance on storing connection strings.
                optionsBuilder.UseSqlServer(connectionString);
            }
        }
    }

Program.cs

using System;
using System.Linq;
using System.IO;
using Microsoft.Extensions.Configuration;

class Program
{
    static void Main(string[] args)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json");

        var configuration = builder.Build();

        using (var db = new MyDbContext(configuration.GetConnectionString("MyDb")))
        {
          //Todo use db
        }
    }
}

project.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="2.0.0" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="2.0.0" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="2.0.0" />
    <PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="2.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration" Version="2.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="2.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration.FileExtensions" Version="2.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="2.0.0" />
  </ItemGroup>
  <ItemGroup>
    <DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="2.0.0" />
    <DotNetCliToolReference Include="Microsoft.EntityFrameworkCore.Tools.DotNet" Version="2.0.0" />
  </ItemGroup>
</Project>