Group docker digests renovate tasks (#33058)

* Group docker digests renovate tasks

* Change group name for testcontainers docker digests

Author: t3chguy

.github/renovate.json

@@ -2,6 +2,14 @@
     "$schema": "https://docs.renovatebot.com/renovate-schema.json",
     "extends": ["github>matrix-org/renovate-config-element-web"],
     "postUpdateOptions": ["pnpmDedupe"],
+    "packageRules": [
+        {
+            "groupName": "testcontainers docker digests",
+            "groupSlug": "{{manager}}-docker-digests",
+            "matchManagers": ["regex"],
+            "matchPackagePatterns": ["*"]
+        }
+    ],
     "customManagers": [
         {
             "customType": "regex",