e2e: tidy up docker-compose file

 * Use double quotes in docker-compose.yml to help with consistency

 * Reorder some blocks to help with readability and maintainability

 * Pin all images to their SHA256 digests

 * Add healthcheck to op-geth-l2 so that op-node/op-proposer only
   start after op-geth-l2 has generated /l2configs/rollup.json

 * Remove `e2e/deployments` and remove the volume from op-geth-l2