renovate: Enable docker manager

Turbo87 · Turbo87 · commit 6a3f41e610ce · 2021-10-18T21:51:22.000+02:00
It would be useful to keep e.g. the `frontend.Dockerfile` up-to-date using this
diff --git a/.github/renovate.json b/.github/renovate.json
@@ -8,9 +8,6 @@
         ":prHourlyLimitNone",
         ":semanticCommitsDisabled"
     ],
-    "docker": {
-        "enabled": false
-    },
     "js": {
         "labels": ["A-frontend"],
         "reviewers": ["locks", "pichfl", "Turbo87"],
