contextfactory.cs

public class BloggingContextFactory : IDesignTimeDbContextFactory<BloggingContext>
{
    public BloggingContext CreateDbContext(string[] args)
    {
        var optionsBuilder = new DbContextOptionsBuilder<BloggingContext>();
        optionsBuilder.UseSqlServer(Environment.GetEnvironmentVariable("SqlConnectionString"));

        return new BloggingContext(optionsBuilder.Options);
    }
}