ci: fix workflow permissions

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

.github/workflows/docs-release.yml

@@ -23,6 +23,9 @@ jobs:
   open-pr:
     runs-on: ubuntu-24.04
     if: ${{ (github.event.release.prerelease != true || github.event.inputs.tag != '') && github.repository == 'docker/buildx' }}
+    permissions:
+      contents: write
+      pull-requests: write
     steps:
       -
         name: Checkout docs repo