Provide example of Terraform provider caching

[rocky-foreflight]

When @aaron-foreflight and I were reviewing #1, we ran into an issue where state had accumulated from his native version of Terraform in the project working directory, which prevented the testing instructions from succeeding.

To rule out any issues with accumulated state, I suggested we clear the .terraform directory and re-run terraform init before each invocation of terraform plan.

However, @aaron-foreflight made a good point about the extra ~30s we'd spend fetching providers before each Terraform operation.

We discovered that Terraform supports caching plugin providers. And we were able to wire things up via the following service definition:

services:
  terraform:
    image: ghcr.io/foreflight/terraform:1.3.4
    environment:
      - TF_PLUGIN_CACHE_DIR=/root/.terraform.d/plugin-cache
    volumes:
      - ./:/usr/local/src
      - $HOME/.terraform.d/plugin-cache:/root/.terraform.d/plugin-cache
      - $HOME/.aws:/root/.aws:ro
    working_dir: /usr/local/src
    entrypoint: bash

We should consider talking about this in the README.

[aaron-foreflight]

@ahuckabee-foreflight ^