feat: Workflow to build protocol docker image

[sanekmelnikov]

What

An image with contracts sources, build artifacts, and pinned forge binary used for building.
Contract hashes are checked against AllContractsHashes.json
This image can be used by zkstack for consistent forge script runs and for verification.

For now - intended for testing.

Command for local build:

docker buildx build \
  --platform linux/amd64 \
  --pull --no-cache \
  -f contracts/docker/protocol/Dockerfile \
  -t local/protocol:v0.29.0 \
  contracts

Why

• eliminates the need to compile contracts locally (faster)
• ensures consistent and reproducible setups for forge runs (independent from local toolchain)

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.