title: Using ConfigurationBuilder in FunctionsStartup
body: <p>I have an Azure function, and I'm using the DI system in order to register some types; for example:</p>
  
  <pre><code>public class Startup : FunctionsStartup
  {
      public override void Configure(IFunctionsHostBuilder builder)
      {
          builder.Services
              .AddTransient&lt;IMyInterface, MyClass&gt;()
      . . . etc
  </code></pre>
  
  <p>However, I also was to register some data from my environment settings.  Inside the function itself, I can get the <code>ExecutionContext</code>, and so I can do this:</p>
  
  <pre><code>IConfiguration config = new ConfigurationBuilder()
     .SetBasePath(context.FunctionAppDirectory)
     .AddJsonFile("local.settings.json", optional: true, reloadOnChange: true)
     .AddEnvironmentVariables()
     .Build();
  </code></pre>
  
  <p>However, in the FunctionsStartup, I don't have access to the <code>ExecutionContext</code>.  Is there a way that I can either get the ExecutionContext from the FunctionsStartup class or, alternatively, another way to determine the current running directory, so that I can set the base path?</p>