Maintenance: Reconfigure push to github to not use a shell runner, but docker.

(cherry picked from commit 60521b3818b1e01c7089a3a2ce4179620d4af42c)

.gitlab/ci/pre.yml

@@ -86,10 +86,12 @@
 # Executed on a dedicated runner.
 "push to github":
   <<: *template_pre
-  tags:
-    - deploy
   before_script:
-    - '' # no RVM present in deploy ENV
+    - test -d $HOME/.ssh || mkdir $HOME/.ssh
+    - ssh-keyscan github.com >> $HOME/.ssh/known_hosts
+    - eval $(ssh-agent -s)
+    - echo "$PUSH_TO_GITHUB_SSH_PRIVATE_KEY" | ssh-add -
+    - ssh-add -l
   script:
     - git fetch --unshallow
     - script/build/sync_repo.sh git@github.com:zammad/zammad.git