BUG: local_files_dir variable currently defaults to /terraform-data

When using this module without our logan tool you get the following error as the default for the local_files_dir variable is /terraform-data. We should change this to be a default that works no matter how you execute terraform. This should probably be something like ${path.root}/build and we just make a note in the README.md that users may want to add ./build to their .gitignore file (if they haven't already) to avoid committing build files to git etc.

╷
│ Error: Output path error
│ 
│   with module.webapp.module.uptime_monitoring["__default"].module.authentication_proxy[0].module.uptime_check_auth_proxy.data.archive_file.function_archive,
│   on .terraform/modules/webapp.uptime_monitoring.authentication_proxy.uptime_check_auth_proxy/main.tf line 13, in data "archive_file" "function_archive":
│   13: data "archive_file" "function_archive" {
│ 
│ error creating output path: mkdir /teraform-data: read-only file system