kas-container: document how to select image

We improve the documentation of the kas-container to better describe how
to use a custom image. Also, we link between the getting started page
and the kas-container page. Ugly looking links are removed from the
getting started guide to reduce the verbosity (they don't provide much
value anyways).

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/userguide/getting-started.rst

@@ -24,15 +24,13 @@ Usage
 There are (at least) four options for using kas:
 
 - Install it locally via pip to get the ``kas`` command.
-- Use the container image locally. In this case, download the ``kas-container``
-  script from the kas repository and use it in place of the ``kas`` command.
+- Use the container image locally. In this case, download the
+  :doc:`kas-container <kas-container>` script from the kas repository and
+  use it in place of the ``kas`` command.
   The script version corresponds to the kas tool and the kas image version.
 - Use the container image in CI. Specify
   ``ghcr.io/siemens/kas/kas[-isar][:<x.y>]`` in your CI script that requests
-  a container image as runtime environment. See
-  /~https://github.com/orgs/siemens/packages/container/kas%2Fkas/versions and
-  /~https://github.com/orgs/siemens/packages/container/kas%2Fkas-isar/versions for
-  all available images.
+  a container image as runtime environment.
 - Use the **run-kas** wrapper from this directory. In this case,
   replace ``kas`` in the examples below with ``path/to/run-kas``.
 

docs/userguide/kas-container-description.inc

@@ -3,5 +3,8 @@ It gives fine grained control over the data that is mapped into the build and
 decouples the build environment from the host system. The wrapper also takes care of
 mounting the necessary directories and setting up the environment variables.
 
-As container backends, Docker and Podman are supported.
-To force the use of podman over docker, set ``KAS_CONTAINER_ENGINE=podman``.
+By default ``kas-container`` uses the official images provided by the kas project:
+``ghcr.io/siemens/kas/kas[-isar]:<version>``. To specify your own image set the
+``KAS_CONTAINER_IMAGE`` environment variable. As container backends, Docker and
+Podman are supported. To force the use of podman over docker, set
+``KAS_CONTAINER_ENGINE=podman``. For details, see :ref:`env-vars-label`.