feat: make it easier to understand when/how to configure custom runner images

config.yml.example

@@ -23,6 +23,11 @@ generator:
 #  spacelift:
 #    manage_state: true
 
+#  # Custom runner to use for stacks with custom runner images configured
+#  # See customizing the runner image: https://docs.spacelift.io/integrations/docker#customizing-the-runner-image
+#  # Default: N/A, must be provided (however, not always used. If it is used and not provided an exception will be raised during code generation)
+#  custom_runner_image: public.ecr.aws/spacelift/runner-terraform
+
   extra_vars:
     foo: bar # "{{ extra_vars.foo }}" in a template will be replaced by "bar"
 

spacemk/commands/generate.py

@@ -21,7 +21,9 @@ def default(value, _default):
             "manage_state": default(config.get("generator.spacelift.manage_state"), True)
         }, "github": {
             "custom_app": default(config.get("generator.github.custom_app"), False)
-        }
+        },
+        "custom_runner_image": default(config.get("generator.custom_runner_image"),
+                                       "SPACELIFT_DEFAULT_INVALID"),
     }
 
     generator = Generator()

spacemk/templates/base.tf.jinja

@@ -95,7 +95,10 @@ resource "spacelift_stack" "{{ stack._relationships.space._migration_id }}_{{ st
   {% block stack_arguments_extra scoped %}{% endblock %}
 
   {% if stack.terraform.workflow_tool == "CUSTOM" %}
-  {{ argument("runner_image ", extra_vars.custom_terraform_runner_image ~ ":" ~ stack.terraform.version, required=True) }}
+  {% if generation_config.custom_runner_image == "SPACELIFT_DEFAULT_INVALID" %}
+  {% raise "generator.custom_runner_image is required for stacks with custom Terraform workflow tool." %}
+  {% endif %}
+  {{ argument("runner_image ", generation_config.custom_runner_image ~ ":" ~ stack.terraform.version, required=True) }}
   {% endif %}
 
   {% if stack.vcs.provider == "github_custom" %}