CI: Use buildah instead of docker

.gitlab-ci.yml

 ---
 default:
-  image: 'docker:19.03.13'
+  image: 'quay.io/containers/buildah'
+  before_script:
+    - 'buildah login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" "$CI_REGISTRY"'
 
 stages:
   - 'sync'
 
 'Sync gentoo/stage3:amd64-systemd':
   stage: 'sync'
-  tags: ['docker']
   script:
-    - 'docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY'
-    - 'docker pull gentoo/stage3:amd64-systemd'
-    - 'docker tag gentoo/stage3:amd64-systemd $CI_REGISTRY_IMAGE:amd64-systemd'
-    - 'docker push $CI_REGISTRY_IMAGE:amd64-systemd'
+    - 'buildah pull gentoo/stage3:amd64-systemd'
+    - 'buildah tag gentoo/stage3:amd64-systemd "${CI_REGISTRY_IMAGE}:amd64-systemd"'
+    - 'buildah push "${CI_REGISTRY_IMAGE}:amd64-systemd"'