Expose a few more configurable items and add input validators

variable.image_name.tf

@@ -1,6 +1,9 @@
 variable "image_name" {
   description = "The image ID or slug to use for the droplet"
-  default     = "fedora-42-x64"
   type        = string
-}
 
+  validation {
+    condition     = can(regex("^[a-zA-Z0-9-]+$", var.image_name))
+    error_message = "The image name must be a valid slug or ID."
+  }
+}