gitlab-ci: Switch the container of the 'check-patch' & 'check-dco' jobs

The 'check-patch' and 'check-dco' jobs only need Python and git for
checking the patches, so it's not really necessary to use a container
here that has all the other build dependencies installed. By using a
lightweight Alpine container, we can improve the runtime here quite a
bit, cutting it down from ca. 1:30 minutes to ca. 45 seconds.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220516082310.33876-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.gitlab-ci.d/static_checks.yml

@@ -1,12 +1,13 @@
 check-patch:
   stage: build
-  image: $CI_REGISTRY_IMAGE/qemu/centos8:latest
-  needs:
-    job: amd64-centos8-container
+  image: python:3.10-alpine
+  needs: []
   script:
     - .gitlab-ci.d/check-patch.py
   variables:
     GIT_DEPTH: 1000
+  before_script:
+    - apk -U add git perl
   rules:
     - if: '$CI_PROJECT_NAMESPACE == "qemu-project"'
       when: never
@@ -15,12 +16,13 @@ check-patch:
 
 check-dco:
   stage: build
-  image: $CI_REGISTRY_IMAGE/qemu/centos8:latest
-  needs:
-    job: amd64-centos8-container
+  image: python:3.10-alpine
+  needs: []
   script: .gitlab-ci.d/check-dco.py
   variables:
     GIT_DEPTH: 1000
+  before_script:
+    - apk -U add git
   rules:
     - if: '$CI_PROJECT_NAMESPACE == "qemu-project" && $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
       when: never