Docker has made this much much easier than it used to be, and the ease of publishing images has led to many SaaS companies providing "dockerized" versions of their services.

I write a lot of Terraform to manage internal infrastructure with dependencies on other internal and external infrastructure that's outside of my control. I define a module parameter for each dependency (usually with a default value that points to the real service URL) and then create a second module that uses Docker to spin up and configure local instances of each dependency and overrides the corresponding parameter.

This makes local testing very straightforward: `terraform -chdir=tests apply`