ceph-dev-pipeline: Add a README

[djgalloway]

Documents how ceph-dev-pipeline actually works. No functional changes — this adds ceph-dev-pipeline/README.md only.

What it covers

• Stage flow — source distribution → matrix (node / checkout ceph-build / copy artifacts / existence checks / builder container / build / upload / container / registry upload), and the fact that matrix cells run on CentOS 9 or Noble hosts regardless of the target distro.
• Where scripts come from — the two independent sources in a cell's workspace, which is the part that trips people up:

  • ./scripts/ from the ceph/ceph-build clone at $CEPH_BUILD_BRANCH (upload/shaman/pulp/podman glue)

  • ./dist/ceph/ extracted from the source tarball produced by the setup job (build-with-container.py, Dockerfile.build, install-deps.sh, debian/, container/)

    So everything running inside the build container comes from the branch being built, not from a checkout.

• The builder container's base OS image — not pinned anywhere in ceph-build. Dockerfile.build is FROM $DISTRO, where DISTRO is a --build-arg set from build-with-container.py's DefaultImage map (centos9 → quay.io/centos/centos:stream9, rocky10 → docker.io/rockylinux/rockylinux:10, noble → docker.io/ubuntu:24.04, etc.). Includes the tag scheme, the /ceph bind mount, and why REGISTRY_AUTH_FILE is set.
• Builder image vs. runtime Ceph dev image — the two are easy to conflate; the latter is built by scripts/build_container → container/build.sh with its own hardcoded FROM_IMAGE map.
• Why SETUP_JOB exists — build-with-container.py landed in ceph.git in Aug 2024 and wasn't backported to reef until Feb 2025 (ceph#61683), so reef branches had to go through ceph-dev-new-setup + ceph-dev-new instead. ceph-trigger-build routes between the two on the CEPH-BUILD-JOB trailer. ceph-source-dist became the pipeline's default setup job in 6e1f737 with ceph-dev-new-setup kept as an option; the two are interchangeable because both archive the same dist/** layout the copy-artifacts stage filters on.
• Key parameters table.

Notes for reviewers

Content was derived by reading the Jenkinsfile, the JJB definition, scripts/, and ceph.git's build-with-container.py / Dockerfile.build / container/Containerfile — worth a sanity check on the base-image table and the SETUP_JOB history in particular.