Benchmark the built Docker image against the released one

[vharseko]

Adds a "Build vs Release" performance benchmark to the Docker build jobs in
build.yml. On every push/PR, after the existing functional smoke tests, the
freshly built OpenDJ image is benchmarked against the latest released
image and the comparison is published in the job summary — so performance
regressions are caught at build time.

This builds on the OpenDJ-vs-OpenLDAP benchmark already on features/performance
(reusing benchmark.jmx and the now-generic summary.sh).

What's added

File	Purpose
.github/benchmark/compare-opendj.sh	Reusable helper: benchmark two OpenDJ images and emit the comparison
.github/workflows/build.yml	Wire the benchmark into build-docker and build-docker-alpine

How it works

compare-opendj.sh <A_name> <A_image> <B_name> <B_image> (env THREADS=200,
DURATION=150):

• Installs ldap-utils/jq and Apache JMeter (cached).
• For each image, sequentially (one container at a time on port 1389): start
  the OpenDJ container, wait for readiness, seed ou=People, capture the version
  (fullVendorVersion), run benchmark.jmx, stop the container.
• Renders the comparison via the generic summary.sh
  (<name> <statistics.json> <version> <image> ×2) into $GITHUB_STEP_SUMMARY:
  versions, totals, per-operation p99 latency, and QuickChart bars.

Both sides are OpenDJ, so no per-server hashing/index setup is needed — identical
product ⇒ identical default password scheme ⇒ hashing parity is automatic.

Wiring in build.yml

Both build-docker and build-docker-alpine gain:

• a sparse actions/checkout of .github/benchmark (these jobs have no checkout,
  and the build artifact does not include those files);
• a JMeter cache step;
• a benchmark step after the functional tests:
  • build-docker — Build = localhost:5000/${GITHUB_REPOSITORY,,}:${release_version}
    vs Release = openidentityplatform/opendj:latest;
  • build-docker-alpine — Build-alpine = …:${release_version}-alpine
    vs Release-alpine = openidentityplatform/opendj:alpine.

Isolation

build-docker and build-docker-alpine are separate jobs, each on its own
ephemeral runner (own registry:3, ports and containers), so they never contend.
Within a job the benchmark runs after the functional tests (whose containers
are already killed) and uses one OpenDJ container at a time on port 1389.

Notes

• Load profile is 200 threads / 150 s per image.
• Validated with actionlint and bash -n; full end-to-end runs only on CI.