Documents environment variables present in workers builds

Author: Maximo-Guk

src/content/docs/workers/ci-cd/builds/configuration.mdx

@@ -78,4 +78,22 @@ It is recommended to consistently use the same API token across all uploads and
 
 [Static assets](/workers/static-assets/) and [frameworks](/workers/framework-guides/) are now supported in Cloudflare Workers. Learn to set up Workers projects and the commands for each framework in the framework guides:
 
-<DirectoryListing folder="workers/framework-guides" maxDepth={4}/>
+<DirectoryListing folder="workers/framework-guides" maxDepth={4} />
+
+## Environment variables
+
+If your project makes use of environment variables to build your site, you can provide custom environment variables:
+
+1. Log in to the [Cloudflare dashboard](https://dash.cloudflare.com) and select your account.
+2. In **Account Home**, select **Workers & Pages**.
+3. In **Overview**, select your Worker.
+4. Select **Settings** > **Environment variables**.
+
+The following system environment variables are injected by default (but can be overridden):
+
+| Environment Variable    | Injected value                        | Example use-case                                                                        |
+| ----------------------- | ------------------------------------- | --------------------------------------------------------------------------------------- |
+| `WORKERS_CI`            | `1`                                   | Changing build behaviour when run on Workers Builds versus locally                      |
+| `WORKERS_CI_BUILD_UUID` | `<build-uuid-of-current-build>`       | Passing the Build UUID along to custom workflows                                        |
+| `WORKERS_CI_COMMIT_SHA` | `<sha1-hash-of-current-commit>`       | Passing current commit ID to error reporting, for example, Sentry                       |
+| `WORKERS_CI_BRANCH`     | `<branch-name-of-current-deployment>` | Customizing build based on branch, for example, disabling debug logging on `production` |