Cross compile docker for arm and arm64

.gitlab-ci.yml

 docker-build:
-  # Use the official docker image.
-  image: docker:latest
+  image: gitlab.jonasled.de/jonasled/buildx-docker:latest
   stage: build
   services:
     - docker:dind
   before_script:
+    - docker context create build
+    - docker buildx create build --use
     - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
-  # Default branch leaves tag empty (= latest tag)
-  # All other branches are tagged with the escaped branch name (commit ref slug)
   script:
     - |
       if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
@@ -17,8 +16,7 @@ docker-build:
         tag=":$CI_COMMIT_REF_SLUG"
         echo "Running on branch '$CI_COMMIT_BRANCH': tag = $tag"
       fi
-    - docker build --pull -t "$CI_REGISTRY_IMAGE${tag}" .
-    - docker push "$CI_REGISTRY_IMAGE${tag}"
+    - docker buildx build --platform linux/amd64,linux/arm,linux/arm64 --push --tag "$CI_REGISTRY_IMAGE${tag}" .
   # Run this job in a branch where a Dockerfile exists
   rules:
     - if: $CI_COMMIT_BRANCH