refactor(docker): disable rollback PRs by default

lib/config/definitions.js

@@ -969,6 +969,7 @@ const options = [
       commitMessageTopic: '{{{depName}}} Docker tag',
       prBody: template('prBody', 'docker'),
       major: { enabled: false },
+      rollbackPrs: false,
       digest: {
         branchTopic: '{{{depNameSanitized}}}-{{{currentTag}}}',
         commitMessageExtra: 'to {{newDigestShort}}',

test/workers/repository/updates/__snapshots__/flatten.spec.js.snap

@@ -245,7 +245,7 @@ Array [
     "recreateClosed": false,
     "requiredStatusChecks": Array [],
     "reviewers": Array [],
-    "rollbackPrs": true,
+    "rollbackPrs": false,
     "schedule": Array [],
     "semanticCommitScope": "deps",
     "semanticCommitType": "chore",