fix(docker): build compose plugin from ./cmd, not ./cmd/compose (#803)

* fix(docker): build compose plugin from ./cmd, not ./cmd/compose

The compose-builder stage in Dockerfile was building ./cmd/compose, which
in docker/compose v5.1.2 is `package compose` (cobra command definitions),
not the CLI plugin entry point. `go build -o file pkg` against a non-main
package writes a Go archive; COPY and chmod accept it, but the kernel
cannot exec an ar-format file, so the Docker CLI plugin manager rejected
the binary and surfaced only as `docker: unknown command: docker compose`
in the v0.64.0 release smoke gate. The plugin's actual main package is at
./cmd (cmd/main.go calls plugin.Run from the cli-plugins framework), per
the upstream Makefile in v5.1.2.

Also fix the version ldflag to target the v5 module path: in v5 the module
moved from github.com/docker/compose/v2 to .../v5, so the previous
-X .../v2/internal.Version= silently no-op'd and `docker compose version`
would have reported an empty string even after the build target fix.

Add -trimpath and -s -w to match the upstream Makefile's release flags.

Add an inline ELF-magic sanity check inside the compose-builder stage so
this exact failure mode (non-main package emitting an archive) fails the
build at the producer instead of surfacing 200 lines later. Hex match via
`od -tx1` works on both busybox and GNU coreutils; the named-character
form (`od -c`) renders fields differently across the two.

Add a binary smoke step to the docker-validate PR job that mirrors the
release-time gate, so a regression in either the CLI or compose source
build is caught at PR review time rather than at tag-push time.

* fix(vex): match Trivy purl format for bundled docker/docker version

The compose plugin build target fix exposes a real consequence: now that
compose-builder produces a true ELF binary (rather than a Go archive),
Trivy can read the embedded Go module metadata and correctly reports the
bundled docker/docker dependency as `v28.5.2+incompatible`. The existing
VEX subcomponent ID omitted the `+incompatible` Go-module suffix, so
Trivy's purl matcher rejected the suppression and CVE-2026-34040 surfaced
in the docker-validate scan against the rebuilt image.

Update the @id to the literal version Trivy emits and bump the document
version per OpenVEX spec. The justification (compose acts as a CLI
client, never as a daemon, so the authz-hook path is unreachable) is
unchanged and continues to apply to v28.5.2+incompatible identically.

* docs(security): explain why CVE-2026-34040 cannot be upgraded out

A direct in-build upgrade attempt (`go get github.com/docker/docker@<sha
of docker-v29.3.1>`) fails with:

  invalid version: go.mod has post-v1 module path
  "github.com/moby/moby/v2" at revision f78c987a

moby/moby migrated its Go module path to `github.com/moby/moby/v2` on
the docker-29.x branch. The legacy `github.com/docker/docker` import path
is therefore frozen at v28.5.2+incompatible from Go's perspective; the
proxy.golang.org listing for that module path confirms no v29.x version
is resolvable. compose v5.1.2 (and v5.1.3, the latest tag at the time of
writing) both still import the legacy path, so the bundled docker/docker
library cannot be moved past v28.5.2+incompatible until upstream compose
migrates its imports.

Document the constraint in two places so a future maintainer (or future
me) does not re-attempt the same `go get` and arrive at the same dead
end:

- Dockerfile compose-builder stage: comment block above the build step.
- security/vex/sencho.openvex.json: expand the CVE-2026-34040 statement's
  impact_statement to spell out the upstream module-split blocker. The
  not_affected status (compose runs as a CLI client, never executes the
  daemon authz hook code path) is the principled triage and remains the
  correct OpenVEX call, not a deferred upgrade.

No build or runtime behaviour changes. Pure documentation enrichment.

* fix(security): mirror CVE-2026-34040 VEX statement in .trivyignore

Trivy reports `Some vulnerabilities have been ignored/suppressed` but
still fails the docker-validate scan on CVE-2026-34040, because the
OpenVEX statement's product identifier `pkg:oci/sencho` cannot match
the build-local image tag `sencho:pr-test`. Trivy does not generate an
OCI purl for images that lack a registry digest (aquasecurity/trivy
issue 9399), so VEX product matching is a no-op at PR and release scan
time. The dual-layer pattern documented in .trivyignore's header was
designed for exactly this case: VEX is the canonical, attested,
published triage record, and .trivyignore mirrors entries that scan-time
matching cannot resolve, with a comment pointing to the corresponding
VEX justification.

The CVE-2026-34040 OpenVEX statement is unchanged (status=not_affected,
justification=vulnerable_code_not_in_execute_path). The .trivyignore
entry is a build-time scan filter only, with a referencing comment so
auditors land on the VEX file as the source of truth. CVE-2026-39883
already follows this pattern and continues to.

security/vex/sencho.openvex.json

@@ -4,8 +4,8 @@
   "author": "Studio Saelix",
   "role": "Vendor",
   "timestamp": "2026-04-26T00:00:00Z",
-  "last_updated": "2026-04-26T00:00:00Z",
-  "version": 1,
+  "last_updated": "2026-04-27T00:00:00Z",
+  "version": 2,
   "statements": [
     {
       "vulnerability": {
@@ -18,7 +18,7 @@
           "@id": "pkg:oci/sencho",
           "subcomponents": [
             {
-              "@id": "pkg:golang/github.com/docker/docker@v28.5.2",
+              "@id": "pkg:golang/github.com/docker/docker@v28.5.2+incompatible",
               "location": "/usr/local/lib/docker/cli-plugins/docker-compose"
             }
           ]
@@ -26,7 +26,7 @@
       ],
       "status": "not_affected",
       "justification": "vulnerable_code_not_in_execute_path",
-      "impact_statement": "The authz bypass applies to Docker daemon plugin authorization hooks. docker-compose v5.1.2 statically bundles docker/docker v28.5.2 as a library but never acts as a daemon and never runs authorization hooks. Sencho invokes compose only for up/down/ps operations against local user-authored compose files. The daemon authz code path is not reachable."
+      "impact_statement": "The authz bypass affects Docker Engine's daemon-side plugin authorization hook on oversized request bodies. docker-compose v5.1.2 statically bundles docker/docker v28.5.2+incompatible as a client-side library for API types and codecs; it never acts as a daemon and never runs authorization hooks. Sencho invokes compose only for up/down/ps operations against local user-authored compose files. The daemon authz code path is not reachable. Note on remediation: the advisory's 'fixed in 29.3.1' refers to Docker Engine the daemon product. The Go library that contains the corresponding source fix is github.com/moby/moby/v2 (a new module path adopted on the docker-29.x branch), not github.com/docker/docker which compose v5.1.2 still imports. Until upstream compose migrates to the v2 import path the v28.5.2+incompatible bundled library is the only Go-module-resolvable version, and the not_affected status is the principled triage rather than a deferred upgrade."
     },
     {
       "vulnerability": {