Ignore terragrunt cache

This excludes the common/default location from source control

Docs: https://terragrunt.gruntwork.io/docs/features/caching/

Terraform.gitignore

@@ -32,3 +32,7 @@ override.tf.json
 # Ignore CLI configuration files
 .terraformrc
 terraform.rc
+
+# Ignore the default terragrunt cache directory
+# https://terragrunt.gruntwork.io/docs/features/caching/
+.terragrunt-cache