Allow overriding cookiecutter's docker imageName

.. to allow running custom-built images that have
cookiecutter installed alongwith extensions, for
example.

https://cookiecutter.readthedocs.io/en/1.7.2/advanced/template_extensions.html

related to:
https://github.com/backstage/backstage/pull/4956#discussion_r596846365

Signed-off-by: Vinay P <mail@vinayvinay.com>

contrib/docker/cookiecutter-with-jinja2-extensions/README.md

@@ -39,9 +39,19 @@ jinja2_custom_filters_extension==0.0.2
 
 #### Using a Cookiecutter Docker image
 
-If the scaffolder doesn't find a local Cookiecutter, it pulls down the `spotify/backstage-cookiecutter` image. You can create a custom Cookiecutter image based on that, install extensions into it, and use that instead. See for example, the [`Dockerfile`](./Dockerfile) in this directory.
+If the scaffolder doesn't find a local Cookiecutter, it pulls down the `spotify/backstage-cookiecutter` image. You can create a custom Cookiecutter image based on that, install extensions into it, and specify that customised image as an input `imageName` to the `fetch:cookiecutter` action:
 
-**Note:** The [`imageName`](https://github.com/vinayvinay/backstage/blob/37e35b91/plugins/scaffolder-backend/src/scaffolder/stages/templater/cookiecutter.ts#L77) that gets pulled isn't currently configurable. So, for the time being, you might have to customise the `scaffolder-backend` plugin to fetch your customised Cookiecutter Docker image.
+```yaml
+steps:
+  - id: fetch-base
+    name: Fetch Base
+    action: fetch:cookiecutter
+    input:
+      url: https://github.com/spotify/cookiecutter-golang
+      imageName: 'foo/custom-built-cookiecutter-image-with-extensions'
+```
+
+See for example, the [`Dockerfile`](./Dockerfile) in this directory.
 
 ### Instructing Cookiecutter to use the extension
 

plugins/scaffolder-backend/src/scaffolder/actions/builtin/fetch/cookiecutter.test.ts

@@ -117,6 +117,7 @@ describe('fetch:cookiecutter', () => {
         extensions: [
           'jinja2_custom_filters_extension.string_filters_extension.StringFilterExtension',
         ],
+        imageName: 'foo/cookiecutter-image-with-extensions',
       },
     });
 
@@ -130,6 +131,7 @@ describe('fetch:cookiecutter', () => {
         _extensions: [
           'jinja2_custom_filters_extension.string_filters_extension.StringFilterExtension',
         ],
+        imageName: 'foo/cookiecutter-image-with-extensions',
       },
     });
   });

plugins/scaffolder-backend/src/scaffolder/actions/builtin/fetch/cookiecutter.ts

@@ -39,6 +39,7 @@ export function createFetchCookiecutterAction(options: {
     values: JsonObject;
     copyWithoutRender?: string[];
     extensions?: string[];
+    imageName?: string;
   }>({
     id: 'fetch:cookiecutter',
     description:
@@ -83,6 +84,12 @@ export function createFetchCookiecutterAction(options: {
               type: 'string',
             },
           },
+          imageName: {
+            title: 'Cookiecutter Docker image',
+            description:
+              "Specify a custom Docker image to run cookiecutter, to override the default: 'spotify/backstage-cookiecutter'. This can be used to execute cookiecutter with Template Extensions. Used only when a local cookiecutter is not found.",
+            type: 'string',
+          },
         },
       },
     },
@@ -121,6 +128,7 @@ export function createFetchCookiecutterAction(options: {
         ...(ctx.input.values as TemplaterValues),
         _copy_without_render: ctx.input.copyWithoutRender,
         _extensions: ctx.input.extensions,
+        imageName: ctx.input.imageName,
       };
 
       // Will execute the template in ./template and put the result in ./result

plugins/scaffolder-backend/src/scaffolder/stages/templater/cookiecutter.ts

@@ -52,9 +52,10 @@ export class CookieCutter implements TemplaterBase {
     // First lets grab the default cookiecutter.json file
     const cookieCutterJson = await this.fetchTemplateCookieCutter(templateDir);
 
+    const { imageName, ...valuesForCookieCutterJson } = values;
     const cookieInfo = {
       ...cookieCutterJson,
-      ...values,
+      ...valuesForCookieCutterJson,
     };
 
     await fs.writeJSON(path.join(templateDir, 'cookiecutter.json'), cookieInfo);
@@ -74,7 +75,7 @@ export class CookieCutter implements TemplaterBase {
       });
     } else {
       await runDockerContainer({
-        imageName: 'spotify/backstage-cookiecutter',
+        imageName: imageName || 'spotify/backstage-cookiecutter',
         args: [
           'cookiecutter',
           '--no-input',