Refine docker workflow

[nuke-web3]

followup to #48 - the first run of https://github.com/celestiaorg/eq-service/actions/runs/13395095717/ suggests we might:

• enure dockerfile layers are cached
• use a much beefier build machine (cpu)
  • https://docs.github.com/en/actions/using-github-hosted-runners/using-larger-runners/about-larger-runners
  • in house?
• only build once, x86 is all we need
• only publish to one registry
• ...more?