Genesis Generator - Use already built docker image

[jpnovais]

Description

We should use an already built Genesis Generator docker image, e.g ethpandaops/ethereum-genesis-generator

Every time we spin up our local stack, we pay the overhead of building one from scratch which is unnecessary.

Motivation

• Improve efficiency on Local Stack and CI speed.
• Make local stack more friendly to use for builders

Tasks

• Adapt l1 genesis generator to use an already built docker image instead of re-doing everytime

Acceptance criteria

• Not build a docker image everytime we need to generate l1 genesis
• L1_GENESIS_TIME shall not be necessary and image shall calculate it automatically - optional and least.