ci: use ko.build to build docker image

.gitlab-ci.yml

@@ -9,9 +9,10 @@ build:
   variables:
     IMAGE: $CI_REGISTRY_IMAGE
     TAG: $CI_COMMIT_REF_SLUG-$CI_COMMIT_SHORT_SHA
+    KO_DOCKER_REPO: $IMAGE:$TAG
   before_script:
     - echo $CI_REGISTRY_PASSWORD | docker login -u $CI_REGISTRY_USER --password-stdin $CI_REGISTRY
-    - docker info
+    - wget https://github.com/ko-build/ko/releases/latest/download/ko_Linux_x86_64.tar.gz
+    - tar -xzvf ko_Linux_x86_64.tar.gz -C /usr/local/bin
   script:
-    - docker buildx create --name yggdrasil --use
-    - docker buildx build --platform linux/amd64,linux/arm64 -t $IMAGE:$TAG --push .
+    - ko build --platform=linux/arm64,linux/amd64 ./cmd/yggdrasil