add ci script for docker

.gitlab-ci.yml

+docker-build:
+  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 run --rm --privileged docker/binfmt:66f9012c56a8316f9244ffd7622d7c21c1f6f28d
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  script:
+    - |
+      if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
+        tag=""
+        echo "Running on default branch '$CI_DEFAULT_BRANCH': tag = 'latest'"
+      else
+        tag=":$CI_COMMIT_REF_SLUG"
+        echo "Running on branch '$CI_COMMIT_BRANCH': tag = $tag"
+      fi
+    - docker buildx build --platform linux/amd64,linux/arm,linux/arm64 --push --tag "$CI_REGISTRY_IMAGE${tag}" .
+  # Run this job in a branch where a Dockerfile exists
+  rules:
+    - if: $CI_COMMIT_BRANCH
+      exists:
+        - Dockerfile