Update instructions for cross OS compatibility

Author: PhilBastian

docs/testing.md

@@ -45,11 +45,13 @@ Containers built by a PR and stored on GitHub Container Registry can be tested l
 1. [Authenticate to the GitHub Container Registry using a personal access token](https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#authenticating-with-a-personal-access-token-classic).
     - Create a [classic token](https://github.com/settings/tokens). Select the scope for `read:packages`
     - Copy the newly created token text.
-    - **On Windows**
-      - create an Environment Variable to store the token (e.g. GITHUB_TOKEN)
-      - open powershell
-      - run the command `echo $env:GITHUB_TOKEN | docker login ghcr.io -u <your particular email> --password-stdin`
-    - ensure that you get a successful login message. use `docker logout ghcr.io` once the following steps are complete and consider removing the token from github if its no longer needed
+    - Run the following command in a terminal:
+       ```shell
+       docker login ghcr.io
+       ```
+      you will be prompted for a username (your particular.net email) and a password (the token)
+    - ensure that you get a successful login message.
+    - Use `docker logout ghcr.io` once the following steps are complete and consider removing the token from github if its no longer needed
 2. In the terminal, navigate to `[/docs/test-ghcr-tag`](/docs/test/ghcr-tag).
 3. Edit the [`.env` file](/docs/test-ghcr-tag/.env) to specify the PR-based tag (in the form `pr-####`) to test.
 4. Run `docker compose up -d`.