Derive app_name from module path (#568)

When changing the name of the application from the default of `app`,
such as when adding multiple applications to a monorepo, developers
often forget to modify the app_name config value in the app-config
module. This change removes the need for that manual step by obtaining
the app_name from the name of the folder containing the application's
infrastructure code.

infra/app/app-config/main.tf

@@ -1,5 +1,8 @@
 locals {
-  app_name              = "app"
+  # app_name is the name of the application, which by convention should match the name of
+  # the folder under /infra that corresponds to the application
+  app_name = regex("/infra/([^/]+)/app-config$", abspath(path.module))[0]
+
   environments          = ["dev", "staging", "prod"]
   project_name          = module.project_config.project_name
   image_repository_name = "${local.project_name}-${local.app_name}"