add ci script

.gitlab-ci.yml

+build-docker:
+  # Use the official docker image.
+  image: gitlab.jonasled.de/jonasled/buildx-docker:latest
+  retry: 2
+  stage: build
+  services:
+    - docker:dind
+  before_script:
+    - docker context create build
+    - docker buildx create build --use
+    - docker run --rm --privileged docker/binfmt:66f9012c56a8316f9244ffd7622d7c21c1f6f28d
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  # Default branch leaves tagempty (= latest tag)
+  # All other branches are tagged with the escaped branch name (commit ref slug)
+  script:
+    - |
+      if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
+        tag="latest"
+      else
+        tag="$CI_COMMIT_REF_SLUG"
+      fi
+    - docker buildx build --platform linux/arm,linux/arm64,linux/amd64 --push --tag "$CI_REGISTRY_IMAGE:${tag}" .