Docs: Clarify DOWNLOAD_URL when building a docker image locally

doc/build/build_docker_image.md

@@ -14,17 +14,19 @@ The `RELEASE` file is not in this directory, and you must create this file.
 ## Create the `RELEASE` file
 The version details of the package being used are stored in the `RELEASE` file.
-To build your own Docker image, create this file with contents similar to the following.
+To build your own Docker image, create this file in the `docker/` folder with
+contents similar to the following.
 ```plaintext
 RELEASE_PACKAGE=gitlab-ee
 RELEASE_VERSION=13.2.0-ee
-DOWNLOAD_URL=https://example.com/gitlab-ee_13.2.00-ee.0_amd64.deb
+DOWNLOAD_URL_amd64=https://example.com/gitlab-ee_13.2.00-ee.0_amd64.deb
 ```
 - `RELEASE_PACKAGE` specifies whether the package is a CE one or EE one.
 - `RELEASE_VERSION` specifies the version of the package, for example `13.2.0-ee`.
-- `DOWNLOAD_URL` specifies the URL where that package can be downloaded from.
+- `DOWNLOAD_URL_amd64` specifies the URL for amd64 where that package can be downloaded from.
+- `DOWNLOAD_URL_arm64` specifies the URL for arm64 where that package can be downloaded from.
 NOTE **Note:**
 We're looking at improving this situation, and using locally available packages