Automatically pull op-reth docker image in `ExternalNode`

[karim-agha]

Currently we're downloading op-reth image manually in github actions to make it available to bollard in external.rs. We need to automatically pull it if not present locally from within rust using CreateImageOptions.

      - name: Try pulling the image manually
        run: docker pull ghcr.io/paradigmxyz/op-reth:latest