build.yml: benchmark the built image against the released one

Add a "Build vs Release" benchmark to the build-docker and build-docker-alpine
jobs: run the LDAP benchmark against the freshly built image and the latest
released image, and publish the comparison in the job summary to catch
performance regressions per push/PR (200 threads / 150s).

- New reusable helper .github/benchmark/compare-opendj.sh runs benchmark.jmx
  against two OpenDJ images sequentially (one container at a time on port 1389:
  start, seed ou=People, capture version, run, stop) and renders the comparison
  via the generic summary.sh. Both sides are OpenDJ, so no per-server
  hashing/index setup is needed (identical default password scheme).
- Each job gets a sparse `actions/checkout` of .github/benchmark (these jobs have
  no checkout and the build artifact does not include those files) plus a JMeter
  cache. build-docker compares localhost:5000/<repo>:<release_version> ("Build")
  vs openidentityplatform/opendj:latest ("Release"); build-docker-alpine compares
  the corresponding -alpine / :alpine tags.