Have migrations and a service that needs the database?

How do I build a service that needs access to my database while also using migrations, given that the migrations tool "dotnet ef database update" runs my application BuildWebHost before creating the database? The attempt to configure the service needing the database throws an exception because the database doesn't exist and causes the migration command to fail. The database therefore never gets created.

I'm using asp.net core 2 and EF Core 2.

More specifically, running "dotnet ef database update" with a blank database fails with the following error:

An error occurred while calling method 'BuildWebHost' on class 'Program'.Continuing without the application service provider. Error: Cannot open database "MyDb" requested by the login. The login failed. Login failed for user 'MYCOMPUTER\MYNAME'.

This happens because I have built a custom configuration provider backed by my "MyDb" database (with the end goal of binding to it with a custom options class) as per this Microsoft Configuration Tutorial My program.cs looks like:

 public static IWebHost BuildWebHost(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .ConfigureAppConfiguration((builderContext, config) =>
                {
                    var tmpconfig = config.Build();
                    config.AddMyOptionsConfig(options => options.UseSqlServer(tmpconfig.GetConnectionString("My_Database")));
                })
            .UseStartup<Startup>()
            .Build();

and the AddMyOptionsConfig eventually runs:

  public class EFConfigProvider : ConfigurationProvider
    {
     [...]

        // Load config data from EF DB.
        public override void Load()
        {
            var builder = new DbContextOptionsBuilder<MyDbContext>();
            OptionsAction(builder);

            using (var dbContext = new MyDbContext(builder.Options))
            {    
                 // dbContext.Database.EnsureCreated(); // will cause first migration to fail
                 Data = !dbContext.ConfigurationValue.Any() // throws exception
                      ? CreateAndSaveDefaultValues(dbContext)
                      : dbContext.ConfigurationValue.ToDictionary(c => c.Id, c => c.Value);

which throws an SQLException when it attempts to access the database because the database hasn't been created yet.

However, if I try to use dbContext.Database.EnsureCreated() then the initial migration fails because the tables already exist. I thought of trying dbContext.Database.Migrate() instead but as a beginner I'm concerned there might be unintended consequences for a production environment. As such I'd prefer to have control over migrations via the command line tools.

Fundamentally, the problem seems to be that "dotnet ef database update" runs the application startup - BuildWebHost - before it creates the database but the custom configuration provider added in BuildWebHost needs the database to already exist.

How do I solve this dilemma?