Migrations in separate assembly how to avoid hardcoded connection string?

To solve this issue I create a class library only for migration with a DbContext deriving from my DbContext but with hard connected connection string.

using Microsoft.EntityFrameworkCore;

namespace ChatLe.Repository.Identity.SqlServer
{
    public class ChatLeIdentityDbContext: ChatLe.Models.ChatLeIdentityDbContext
    {
        public ChatLeIdentityDbContext()
        {
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=chatle;Trusted_Connection=True;MultipleActiveResultSets=true");
            base.OnConfiguring(optionsBuilder);
        }
    }
}

Then I launch ef tool command like this :

• To add a migration run dotnet ef --startup-project {path to my startup project} migrations add login-activity --context ChatLe.Repository.Identity.SqlServer.ChatLeIdentityDbContext
• To upgrade the database run dotnet ef --startup-project {path to my startup project} database update --context ChatLe.Repository.Identity.SqlServer.ChatLeIdentityDbContext

Read the full sample on my git hub project : https://github.com/aguacongas/chatle/tree/develop/src/ChatLe.Repository.Identity.SqlServer