fix(ci): only push docker image if we have the necessary permissions

We run this workflow in several contexts: `pull_request`, `tags` and `push`. For the events that are not `pull_request`, this property won't be defined, i.e. be `undefined`. Negating that should yield `true` which is what we want: push the image for the `master` branch and on new tags that start with `libp2p-server-*`.

Pull-Request: #4502.

.github/workflows/docker-image.yml

@@ -34,6 +34,6 @@ jobs:
         with:
           context: .
           file: ./misc/server/Dockerfile
-          push: true
+          push: ${{ ! github.event.pull_request.head.repo.fork }} # Only push image if we have the required permissions, i.e. not running from a fork
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}