Docker volume mount removed

[Theo-]

In #59, the volume mount for op-geth got removed. This caused our nodes to restart from zero after pulling the latest.

Flagging in case it wasn't intentional, if it was, please close :)

[wbnns]

@Theo-

Heya, happy new year! Thanks for reporting this! Let me look into this and will report back. 👍

[mdehoog]

hey @Theo-, this was intentional, but open to adding it back in. It was introduced in #49 but removed 5 days later because It is mentioned in the README as an option, but we didn't necessarily want to be opinionated about where the volume should be stored locally.

[danyalprout]

I think mounting a data dir by default seems reasonable, as anyone who restores from a snapshot needs that behavior.

In the PR above, added a volume with a default name, that can be overriden by an env var.