[CI] Use detached step instead of service to run dockerd only after linters

.drone.yml

@@ -25,6 +25,16 @@ steps:
   commands:
     - mdl --style all --warnings .
 
+# use detached image / service to run dockerd
+# services:
+- name: docker
+  image: docker:18.09-dind
+  environment:
+    DOCKER_TLS_CERTDIR: ""
+  command: [ "--storage-driver=vfs", "--tls=false" ]
+  privileged: true
+  detached: true
+
 - name: build image
   image: docker:stable
   environment:
@@ -38,10 +48,3 @@ steps:
   - docker login -u $USERNAME -p $PASSWORD
   - docker image push kleph/beer
 
-services:
-  - name: docker
-    image: docker:18.09-dind
-    environment:
-      DOCKER_TLS_CERTDIR: ""
-    command: [ "--storage-driver=vfs", "--tls=false" ]
-    privileged: true