feat: add a terraform format checker task

README.md

@@ -211,3 +211,26 @@ include:
   - project: 'gitlab-com/gl-infra/common-ci-tasks'
     file: tflint.yml
 ```
+
+### [`terraform-format`](./terraform-format.yml)
+
+Runs [`terraform fmt`](https://www.terraform.io/cli/commands/fmt) to ensure that all Terraform files are correctly formatted.
+
+1. Ensure that the `GL_ASDF_TERRAFORM_VERSION` version for terraform is configured.
+
+```yaml
+stages:
+  - validate
+
+# Not needed if .gitlab-ci-asdf-versions.yml is included...
+variables:
+  GL_ASDF_TERRAFORM_VERSION: ...
+
+include:
+  # Not required, but recommended
+  - local: .gitlab-ci-asdf-versions.yml
+
+  # Ensures that all terraform files are correctly formatted
+  - project: 'gitlab-com/gl-infra/common-ci-tasks'
+    file: terraform-format.yml
+```

terraform-format.yml

+terraform_format:
+  stage: validate
+  needs: []
+  image:
+    name: hashicorp/terraform:${GL_ASDF_TERRAFORM_VERSION}
+    entrypoint: [""]
+  script:
+    - terraform fmt -recursive -diff -check .
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH