Improve docker login instructions

Author: Matthias Koeppe

src/doc/en/developer/portability_testing.rst

@@ -1018,13 +1018,21 @@ The ``docker`` workflows automatically push images to
 GitHub repository.
 
 In order to pull them for use on your computer, you need to first
-visit https://github.com/settings/tokens/new to generate a Personal
-Access Token providing the ``read:packages`` scope.  Then log in using
-your GitHub user name and the Personal Access Token::
+generate a Personal Access Token providing the ``read:packages`` scope
+as follows.  Visit https://github.com/settings/tokens/new (this may
+prompt you for your GitHub password).  As "Note", type "Access
+docker.pkg.github.com"; then in "Select scopes", select the checkbox
+for ``read:packages``.  Finally, push the "Generate token" button at
+the bottom.  This will lead to a page showing your token, such as
+``de1ec7ab1ec0ffee5ca1dedbaff1ed0ddba11``.  Copy this token and paste
+it to the command line::
 
-  $ docker login docker.pkg.github.com
+  $ echo de1ec7ab1ec0ffee5ca1dedbaff1ed0ddba11 | docker login docker.pkg.github.com --username YOUR-GITHUB-USERNAME
+
+where you replace the token by your token, of course, and
+``YOUR-GITHUB-USERNAME`` by your GitHub username.
 
 Now you can pull the image and run it::
 
-  $ docker pull docker.pkg.github.com/mkoeppe/sage/sage-docker-fedora-31-standard-configured:f4bd671
-  $ docker run -it docker.pkg.github.com/mkoeppe/sage/sage-docker-fedora-31-standard-configured:f4bd671 bash
+  $ docker pull docker.pkg.github.com/YOUR-GITHUB-USERNAME/sage/sage-docker-fedora-31-standard-configured:f4bd671
+  $ docker run -it docker.pkg.github.com/YOUR-GITHUB-USERNAME/sage/sage-docker-fedora-31-standard-configured:f4bd671 bash