Adds forking config to the `ethereumoptimism/hardhat` docker image.
This can be configured with the following env vars:

- `FORK_URL` - http provider of the remote node to fetch data from
- `FORK_STARTING_BLOCK` - block to start forking from, if left
  undefined it will use the tip

Without `FORK_URL` set, it will not run in forking mode

Also adds the ability to configure the gas price with:

- `GAS_PRICE` - defaults to 0, which was the previous behavior