renovate: ignore docker/github-builder-experimental

This action has no tagged releases, so Renovate would open a PR for
every new commit. The action is still experimental and frequently
receives commits that would trigger unnecessary update PRs.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Author: vvoland

.github/renovate.json5

@@ -28,5 +28,9 @@
   "ignoreDeps": [
     // We're using the "v2" branch, not the default ("v1") branch.
     "github.com/Graylog2/go-gelf",
+    // This action has no tagged releases, so Renovate would open a PR for
+    // every new commit.
+    // TODO: Remove once there's a tagged release.
+    "docker/github-builder-experimental",
   ]
 }