gitlab/opensbi: Move to docker:stable

gitlab/opensbi: Move to docker:stable

The OpenSBI build has been using docker:19.03.1, which appears to be old
enough that v2 of the manifest is no longer supported.  Something has
started serving us those manifests, resulting in errors along the lines
of

    $ docker build --cache-from $IMAGE_TAG --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA --tag $IMAGE_TAG .gitlab-ci.d/opensbi
    Step 1/7 : FROM ubuntu:18.04
    18.04: Pulling from library/ubuntu
    mediaType in manifest should be 'application/vnd.docker.distribution.manifest.v2+json' not 'application/vnd.oci.image.manifest.v1+json'

This moves to docker:stable, as was suggested by the template.  It also
adds the python3 package via apt, as OpenSBI requires that to build.

Reviewed-by: Bin Meng <bmeng@tinylab.org>
Message-ID: <20230303202448.11911-2-palmer@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>