Clarify variable forwarding with Auto Build CNBs

The fact that Cloud Native Buildpacks support the same kind of CI
variable forwarding was not clear, see
https://gitlab.com/gitlab-org/gitlab/-/issues/220684

doc/topics/autodevops/customize.md

@@ -72,25 +72,20 @@ Avoid passing secrets as Docker build arguments if possible, as they may be
 persisted in your image. See
 [this discussion of best practices with secrets](https://github.com/moby/moby/issues/13490) for details.
 
-## Passing secrets to `docker build`
+## Forward CI variables to the build environment
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/25514) in GitLab 12.3, but available in versions 11.9 and above.
 
-CI environment variables can be passed as
-[build secrets](https://docs.docker.com/develop/develop-images/build_enhancements/#new-docker-build-secret-information) to the `docker build` command by listing them
-by name, comma-separated, in the `AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES`
-variable. For example, to forward the variables `CI_COMMIT_SHA` and `CI_ENVIRONMENT_NAME`,
-set `AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES` to `CI_COMMIT_SHA,CI_ENVIRONMENT_NAME`.
+CI variables can be forwarded into the build environment using the
+`AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES` CI variable.
+The forwarded variables should be specified by name in a comma-separated
+list. For example, to forward the variables `CI_COMMIT_SHA` and
+`CI_ENVIRONMENT_NAME`, set `AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES`
+to `CI_COMMIT_SHA,CI_ENVIRONMENT_NAME`.
 
-CAUTION: **Caution:**
-Unlike build arguments, these variables are not persisted by Docker in the final image,
-though you can still persist them yourself.
-
-In projects:
-
-- Without a `Dockerfile`, these are available automatically as environment
-  variables.
-- With a `Dockerfile`, the following is required:
+- When using Buildpacks, the forwarded variables are available automatically
+  as environment variables.
+- When using a `Dockerfile`, the following additional steps are required:
 
   1. Activate the experimental `Dockerfile` syntax by adding the following code
      to the top of the file:
@@ -304,7 +299,7 @@ applications.
 | `AUTO_DEVOPS_BUILD_IMAGE_CNB_ENABLED`   | When set to a non-empty value and no `Dockerfile` is present, Auto Build builds your application using Cloud Native Buildpacks instead of Herokuish. [More details](stages.md#auto-build-using-cloud-native-buildpacks-beta). |
 | `AUTO_DEVOPS_BUILD_IMAGE_CNB_BUILDER`   | The builder used when building with Cloud Native Buildpacks. The default builder is `heroku/buildpacks:18`. [More details](stages.md#auto-build-using-cloud-native-buildpacks-beta). |
 | `AUTO_DEVOPS_BUILD_IMAGE_EXTRA_ARGS`    | Extra arguments to be passed to the `docker build` command. Note that using quotes won't prevent word splitting. [More details](#passing-arguments-to-docker-build). |
-| `AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES` | A [comma-separated list of CI variable names](#passing-secrets-to-docker-build) to be passed to the `docker build` command as secrets. |
+| `AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES` | A [comma-separated list of CI variable names](#forward-ci-variables-to-the-build-environment) to be forwarded to the build environment (the buildpack builder or `docker build`). |
 | `AUTO_DEVOPS_CHART`                     | Helm Chart used to deploy your apps. Defaults to the one [provided by GitLab](https://gitlab.com/gitlab-org/charts/auto-deploy-app). |
 | `AUTO_DEVOPS_CHART_REPOSITORY`          | Helm Chart repository used to search for charts. Defaults to `https://charts.gitlab.io`. |
 | `AUTO_DEVOPS_CHART_REPOSITORY_NAME`     | From GitLab 11.11, used to set the name of the Helm repository. Defaults to `gitlab`. |