ci: Use Buildah instead of Docker to generate images

The GNOME runners are no more privileged and thus it is not possible to
use Docker from the pipeline. Let's use Buildah instead Docker to fix
the image generation.

.gitlab-ci.yml

@@ -26,7 +26,9 @@ nightly:
         - 'flatpak devel'
 
 fedora rawhide:
-    image: registry.gitlab.gnome.org/gnome/nautilus:latest
+    image:
+        name: registry.gitlab.gnome.org/gnome/nautilus:latest
+        entrypoint: ["/bin/sh", "-c"]
     stage: cross environment
     script:
         - meson _build .
@@ -88,18 +90,19 @@ style check:
 
 update image:
     variables:
-         DOCKER_TLS_CERTDIR: ""
-    image: docker:latest
+         STORAGE_DRIVER: vfs
+         BUILDAH_FORMAT: docker
+         BUILDAH_ISOLATION: chroot
+    image: registry.fedoraproject.org/fedora:33
     stage: image
-    tags:
-        - privileged
-    services:
-        - docker:dind
     script:
-        - docker build --tag $CI_REGISTRY_IMAGE .gitlab/
-        - docker tag $CI_REGISTRY_IMAGE "$CI_REGISTRY_IMAGE:v$CI_JOB_ID"
-        - docker login $CI_REGISTRY -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD
-        - docker push $CI_REGISTRY_IMAGE
+        - dnf install -y buildah runc
+        - sed -i '/^mountopt =.*/d' /etc/containers/storage.conf
+        - buildah bud --tag $CI_REGISTRY_IMAGE -f .gitlab/Dockerfile
+        - buildah tag $CI_REGISTRY_IMAGE "$CI_REGISTRY_IMAGE:v$CI_JOB_ID"
+        - buildah login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+        - buildah push $CI_REGISTRY_IMAGE
+        - buildah push "$CI_REGISTRY_IMAGE:v$CI_JOB_ID"
     when: manual
     only:
         - web