If you comment out: .AddAzureBlobMediaFileSystem() and/or .AddAzureBlobImageSharpCache() in your startup.cs, Umbraco should use the local Media folder.

You could automate this by adding an if statement to the ConfigureServices() method of your Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
     var umbracoBuilder = services.AddUmbraco(_env, _config)
          .AddBackOffice()
          .AddWebsite()
          .AddComposers();

     if (!_env.IsDevelopment())
     {
          umbracoBuilder.AddAzureBlobMediaFileSystem()
               .AddAzureBlobImageSharpCache();
     }

     umbracoBuilder.Build();
}

Here the blob storage middleware is only registered if you are not running the Development environment.

(_env is IWebHostEnvironment, which should already be injected in your constructor)