set MTU to 1400

On hetzner, we need a 1400 MTU, but it seems there is no way to
configure this per host, so we do it here

.gitlab-ci.yml

@@ -10,6 +10,8 @@ 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}