set mtu to 1400

That didn't work, so use whats documented here:
https://github.com/docker-library/docker/issues/103#issuecomment-478619847

.gitlab-ci.yml

 image: docker:latest
 
 services:
-- docker:dind
+  - name: docker:dind
+    # MTU 1400 for hetzner vSwitch
+    command: ["--mtu=1400"]
 
 stages:
 - build
@@ -10,8 +12,6 @@ variables:
   IMAGE_TAG: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME
   DOCKER_HOST: tcp://docker:2375
   DOCKER_TLS_CERTDIR: ""
-  # MTU 1400 for hetzner vSwitch
-  DOCKER_OPTS: "--mtu=1400"
 
 cache:
   key: ${CI_PROJECT_ID}