tell gitlab ci to use the shell executor

this is actually a bit sad since I don't want to specify in my repo which runner to use. It should be possible with docker and shell executor

.gitlab-ci.yml

-image: texlive/texlive:latest
-
 pdf:
   script:
       # use settings from .latexmkrc
@@ -10,3 +8,5 @@ pdf:
     paths:
     - "Felix-Hilsky-master-thesis.pdf"
     expire_in: 4 weeks
+  tags:
+    - shell