ci: list pushed image URLs in build-container completion mail (#2098)

The verification team consumes the containers `nixl-ci-build-container`
publishes. `nixl-ci-build-llm-container` already names its images in the
completion mail; this brings the same behaviour to
`nixl-ci-build-container`.

The image tag was resolved per-arch inside the matrix pods, so
`pipeline_stop` had no way to know it. The tag resolution moves to
`pipeline_start` instead, where it happens once and is shared by both
the build steps and the mail.

- `pipeline_start` resolves the NIXL commit (from ci-demo's
`GIT_COMMIT`) and the UCX commit (one `git ls-remote`, or truncation
when `UCX_VERSION` is already a hash), and exports `NIXL_SHA`, `UCX_SHA`
and `IMAGE_TAG_BASE`.
- `Add Version Info` consumes `IMAGE_TAG_BASE` instead of re-deriving
both SHAs per arch, dropping ~18 lines.
- Both build steps now use `UCX_SHA` rather than re-resolving
`UCX_VERSION`. Each arch cloned UCX independently before, so a branch
moving mid-build could ship two arches built from different UCX commits
under a single tag.
- `pipeline_stop` lists the pushed image URLs, plus the `-latest` tags
when `UPDATE_LATEST` is set. The list is included only on `SUCCESS` — a
failed run may not have pushed, and a link to a missing image is worse
than no link.
- `.ci/docs/ci-overview.md` updated to match.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Documentation**
- Clarified that NIXL and UCX revisions are resolved once per build and
reused across architectures.
- Documented image tag generation and successful completion emails with
pushed image URLs and optional latest tags.

- **Bug Fixes**
- Improved build consistency by centralizing revision metadata and
validating UCX revision resolution.
- Ensured successful-build emails include the appropriate image tags and
are not sent for failed builds.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Signed-off-by: NirWolfer <nwolfer@nvidia.com>