add docs note about quoting string variables on the command line

website/pages/guides/hcl/variables.mdx

@@ -88,6 +88,79 @@ $ packer build \
 Once again, setting variables this way will not save them, and they'll
 have to be input repeatedly as commands are executed.
 
+
+If you plan to assign variables via the command line, we strongly recommend that
+you at least set a default type instead of using empty blocks; this helps the
+HCL parser understand what is being set.
+
+For example:
+
+```hcl
+variable "pizza" {
+  type = string
+}
+
+source "null" "example" {
+    communicator = "none"
+}
+
+build {
+  sources = [
+    "source.null.example"
+  ]
+  provisioner "shell-local" {
+    inline = ["echo $PIZZA"]
+    environment_vars = ["PIZZA=${var.pizza}"]
+  }
+}
+```
+
+If you call the above template using the command
+
+```sh
+packer build -var pizza=pineapple shell_local_variables.pkr.hcl
+```
+
+then the Packer build will run successfully. However, if you define the variable
+using an empty block, the parser will not know what type the variable is, and it
+cannot infer the type from the command line, as shown in this example:
+
+```hcl
+variable "pizza" {}
+
+source "null" "example" {
+    communicator = "none"
+}
+
+build {
+  sources = [
+    "source.null.example"
+  ]
+  provisioner "shell-local" {
+    inline = ["echo $PIZZA"]
+    environment_vars = ["PIZZA=${var.pizza}"]
+  }
+}
+```
+The above template will result in the error:
+
+```
+Error: Variables not allowed
+
+   on <value for var.pizza from arguments> line 1:
+   (source code not available)
+
+Variables may not be used here.
+```
+
+You can work around this either by quoting the variable on the command line, or
+by adding the type to the variable block as shown in the previous example.
+Setting the expected type is the more resilient option.
+
+```sh
+packer build -var 'pizza="pineapple"' shell_local_variables.pkr.hcl
+```
+
 #### From a file
 
 To persist variable values, create a file and assign variables within