fix: removing legacy-peer-deps flag and adding Docker build kit in readme.md

[lgahdl]

Description

Addresses two developer experience issues identified during re-testing. Neither is a blocker but both affect the out-of-the-box setup experience.

Changes

• Add # syntax=docker/dockerfile:1 frontend pragma to modules/services/Dockerfile, modules/services/playground/Dockerfile.cowswap, and
  modules/services/playground/Dockerfile.explorer — this enables BuildKit automatically, making --mount=type=cache work without requiring
  DOCKER_BUILDKIT=1 to be set manually
• Document Docker 23+ as the minimum version in the README prerequisites, with a note that older versions need DOCKER_BUILDKIT=1
• Add "overrides": { "ethers": "^6.9.0" } to the root package.json to resolve the peer dependency conflict between ethers@^6 (devDependencies) and
  @cowprotocol/contracts / @cowprotocol/app-data (which declare ethers@^5 as a peer dep). Neither package is imported in TypeScript — they are
  Foundry/Solidity dependencies only — so forcing ethers v6 across the tree is safe
• Remove --legacy-peer-deps from the npm install step in the README

How to test

1. Run npm install from the repo root — should succeed with no ERESOLVE error and no flags required
2. Run docker build against any of the three modified Dockerfiles without setting DOCKER_BUILDKIT=1 — build should succeed and --mount=type=cache
   layers should be recognized