ci(renovate): Assign all MRs to Sheogorath

This patch adjusts renovate to automatically assign MRs to me, in order
to utilise the GitLab todo feature a bit better.

renovate.json

@@ -4,7 +4,8 @@
     "config:base",
     "docker:enableMajor",
     ":prHourlyLimitNone",
-    "regexManagers:dockerfileVersions"
+    "regexManagers:dockerfileVersions",
+    ":assignAndReview(sheogorath)"
   ],
   "prConcurrentLimit": 20,
   "semanticCommits": "enabled",