Create 4-custom-transformers.md

circle_ci/4-custom-transformers.md

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+# Using custom transformers to customize Valet's behavior
+
+In this lab you will build upon the `dry-run` command to override Valet's default behavior and customize the converted workflow using "custom transformers". Custom transformers can be used to:
+
+1. Convert items that are not automatically converted.
+2. Convert items that were automatically converted using different actions.
+3. Convert environment variable values differently.
+4. Convert references to runners to use a different runner name in Actions.
+
+## Prerequisites
+
+1. Followed the steps [here](./readme.md#configure-your-codespace) to set up your GitHub Codespaces environment and start you GitLab server.
+2. Completed the [configure lab](./1-configure.md#configuring-credentials).
+3. Completed the [dry-run lab](./3-dry-run.md).
+
+## Perform a dry-run
+
+You will be performing a `dry-run` command to inspect the workflow that is converted by default. Run the following command within the codespace terminal:
+
+```bash
+# ADD_COMMAND
+```
+
+The converted workflow that is generated by the above command can be seen below:
+
+<details>
+  <summary><em>Converted workflow 👇</em></summary>
+
+```yaml
+# ADD_YAML
+```
+
+</details>
+
+_Note_: You can refer to the previous [lab](./3-dry-run.md) to learn about the fundamentals of the `dry-run` command.
+
+## Custom transformers for an unknown step
+
+ADD_CONTENT_OR_REMOVE
+
+## Custom transformers for environment variables
+
+ADD_CONTENT
+
+That's it! Congratulations, you have overridden Valet's default behavior by customizing the conversion of:
+
+- UPDATE_LIST_HERE
+- Environment variables
+
+## Next lab
+
+[Perform a production migration of a CircleCI pipeline](5-migrate.md)