feat: use already-built image in Dockerfile of l1-node-genesis-generator

[jonesho]

This PR implements issue(s) #638

• use external docker image ethpandaops/ethereum-genesis-generator in Dockerfile of l1-node-genesis-generator
• get genesis time automatically in the docker container and make L1_GENESIS_TIME optional in the docker compose file

Checklist

• I wrote new tests for my new core changes.
• I have successfully ran tests, style checker and build against my new changes locally.
• I have informed the team of any breaking changes if there are any.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.38%. Comparing base (0eb6eb4) to head (57bceba).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #669      +/-   ##
============================================
- Coverage     68.43%   68.38%   -0.06%     
  Complexity     1185     1185              
============================================
  Files           323      323              
  Lines         13130    13131       +1     
  Branches       1318     1319       +1     
============================================
- Hits           8986     8979       -7     
- Misses         3593     3600       +7     
- Partials        551      552       +1     

Flag	Coverage Δ		*Carryforward flag
hardhat	98.52% <ø> (ø)		Carriedforward from 18a7faf
kotlin	66.03% <ø> (-0.07%)	⬇️

*This pull request uses carry forward flags. Click here to find out more.

see 4 files with indirect coverage changes