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>

.gitlab-ci.d/opensbi.yml

@@ -42,9 +42,9 @@
 docker-opensbi:
   extends: .opensbi_job_rules
   stage: containers
-  image: docker:19.03.1
+  image: docker:stable
   services:
-    - docker:19.03.1-dind
+    - docker:stable-dind
   variables:
     GIT_DEPTH: 3
     IMAGE_TAG: $CI_REGISTRY_IMAGE:opensbi-cross-build

.gitlab-ci.d/opensbi/Dockerfile

@@ -15,6 +15,7 @@ RUN apt update \
         ca-certificates \
         git \
         make \
+	python3 \
         wget \
     && \
     \