[s2x]

What changed

The five FDB services repeated the image, coordinator string, environment
entries, entrypoint, restart policy, healthcheck and common volume mounts, and
the CI e2e job still used the legacy docker-compose binary plus a duplicate
tenant-mode step.

• docker-compose.yml: shared x-fdb-* anchors for the image, coordinator
  string (FDB_CLUSTER_FILE_CONTENTS), environment map, entrypoint, restart
  policy, healthcheck and the two common volume items. All five FDB services
  use them; the coordinator string now appears once.
• fdb-config and php entrypoints write the cluster file from
  FDB_CLUSTER_FILE_CONTENTS instead of a hardcoded string.
• .github/workflows/ci.yml e2e: legacy install removed, every call uses
  docker compose, and the duplicate tenant-mode step is replaced by a bounded
  wait for the fdb-config sentinel healthcheck.
• CHANGELOG.md records the change under [Unreleased]/Changed.

Closes #166

How it was tested

• docker compose config --format json on master vs HEAD: identical
  effective FDB_PORT, FDB_BG_URL, cluster contents, restart policy,
  healthcheck and volume mounts for all five FDB services; only the intended
  entrypoint/env changes differ.
• grep -c 'docker:docker@' docker-compose.yml = 1; fdb-config and php
  write the exact string plus one newline.
• Fresh docker compose up -d of the six services: available after 7 s,
  fdb-config healthy after 7 s, tenant_mode: optional_experimental;
  docker compose ps -q, exec and down -v work.
• make test: 926 tests, 2500 assertions, 6 skipped.
  make test-integration: 309 tests, 1233 assertions, 6 skipped.
• docker compose config -q, bin/lint.sh, actionlint, git diff --check
  pass.

Checklist

• Tests added or updated, and all checks pass
• CHANGELOG.md updated under [Unreleased]
• Documentation updated
• Everything is written in English
• The PR title is a Conventional Commit (chore: ...)