Set benchmark job to use dedicated database server

.gitlab-ci.yml

@@ -36,10 +36,7 @@ benchmark:
   when: manual
   variables:
     MIX_ENV: benchmark
-  services:
-  - name: postgres:9.6
-    alias: postgres
-    command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
+    DB_HOST: pleromaci-db1.your.org
   script:
     - mix deps.get
     - mix ecto.create