fix(ci): skip the BuildKit check whose warning embedded this Dockerfile in CI output

v1.10.1 (run 707) failed with ZERO failing steps: every step Success, `smoke`
printing "101 passed, 0 failed", `smoke-studio` "104 passed, 0 failed", both
jobs red. The clipboard fix from f3b3748 worked exactly as predicted; this is a
second, independent defect that run 704 had masked.

CHAIN, measured end to end rather than reasoned:

`ARG BASE_IMAGE` has no default on purpose (the two-phase build always supplies
it), which trips BuildKit's InvalidDefaultArgInFrom check. buildx attaches that
warning's source context to the build metadata as
buildx.build.warnings[].sourceInfo.data — the ENTIRE Dockerfile, base64, on ONE
line. docker/build-push-action writes that metadata to $GITHUB_OUTPUT as a
`name<<ghadelimiter_<uuid>` heredoc. Gitea's act_runner truncates any single
line at exactly 65536 chars, so the closing delimiter was cut off:

  invalid format delimiter 'ghadelimiter_...' not found before end of file

and the runner failed the job while attributing it to no step at all.

  v1.9.4  run 695 SUCCESS: longest metadata line 56355, 0 delimiter errors
  v1.10.0 run 704 failed : longest metadata line 65536, 1 delimiter error
  v1.10.1 run 707 failed : longest metadata line 65536, 1 delimiter error

65536 = 2^16: cut AT the cap, not merely long. Independent route via file size
rather than log parsing: 42251 B at v1.9.4 (base64 56335, matching the log) vs
50034 B now (base64 66712, truncated). The cap corresponds to a 49152 B
Dockerfile, so this cycle's comment growth crossed it by 882 B.

Located by asking which top-level metadata key CONTAINS the base64, instead of
assuming: it is buildx.build.warnings -> sourceInfo -> data in BOTH runs.

THIS IS THE SECOND FIX FOR THIS BUG. The first, `provenance: false` on the two
smoke build steps, was committed as 784fad7 and is REVERTED here: a real buildx
on another host showed provenance metadata present in both modes with a longest
string of 71 chars, i.e. the base64 was never in provenance. Shipping it would
have left the release broken a third time while looking like a fix.

Also rejected, each measured: floating action tags moving (act action-bundle
hashes byte-identical between runs 695 and 707, all four) and the build-check
annotation text changing (byte-identical).

FIX: `# check=skip=InvalidDefaultArgInFrom` as the FIRST line of
Dockerfile.variant — BuildKit parses `# check=` only before any other line, so
placement is load-bearing. No honest default exists for BASE_IMAGE: `scratch`
would satisfy the linter while being a lie, and would convert today's instant
"invalid reference format" into a failure deep in the build. Verified against
the actual edited file with `docker buildx build --check`: "Check complete, no
warnings found." Zero warnings means no sourceInfo, so the longest metadata line
drops 65536 -> ~1936 and the file's SIZE stops gating CI.

GUARD: scripts/check-dockerfile-directives.sh, wired into BOTH lint.yml and
docker-publish.yml's lint-gate, because a check that gates only `push` lets a
tag regress — the adoption slip that let doc-drift land 27 h after the v1.9.3
tag. It also fails if ARG BASE_IMAGE gains a default, so the directive cannot
rot into guarding a check that can no longer fire. Truth table, exit codes:
present 0, removed 1, demoted to line 2 → 1, ARG defaulted 1, file missing 2
(a gate that cannot run must not pass).

Release renamed v1.10.1 -> v1.10.2 with both failed tags left standing as
tombstones. Docs swept again (README "since" marker, Dockerfile decision
comments) because CI reads them from the TAG.

Gates: doc-drift 23 OK / 0 DRIFT; base-hash, workflow-shell, skill-floor,
lint-shell (17 files now), dockerfile-directives all rc=0.

.gitea/workflows/docker-publish.yml

@@ -222,6 +222,13 @@ jobs:
       - name: Components the next build would bake differently are named in the CHANGELOG
         run: bash scripts/check-doc-drift.sh
 
+      # Runs HERE as well as in lint.yml deliberately. A guard that gates only
+      # `push` lets a tag regress — the adoption slip that let doc-drift land
+      # 27 h after the v1.9.3 tag. This one protects the tag build from the
+      # zero-failing-step failure mode that killed v1.10.0 and v1.10.1.
+      - name: Dockerfile.variant still opens with its BuildKit check directive
+        run: bash scripts/check-dockerfile-directives.sh Dockerfile.variant
+
   resolve-versions:
     # Gated: a defective tree must not reach a 46-minute base build.
     needs: [lint-gate]
@@ -593,28 +600,6 @@ jobs:
           platforms: linux/amd64
           push: false
           load: true
-          # provenance: false is LOAD-BEARING, not hygiene. buildx writes a
-          # provenance attestation into the metadata it hands back, and that
-          # metadata embeds the ENTIRE Dockerfile as one base64 "data" field on a
-          # single line. build-push-action writes the metadata to $GITHUB_OUTPUT
-          # as a `name<<ghadelimiter_<uuid>` heredoc, and Gitea's act_runner
-          # truncates any single line at exactly 65536 chars — so once
-          # base64(Dockerfile.variant) crosses 64 KiB the closing delimiter is
-          # cut off and the runner fails the job with
-          #   invalid format delimiter 'ghadelimiter_...' not found before end of file
-          # and NO failing step: every step reports Success, the smoke suite
-          # reports "0 failed", and the job is red anyway.
-          # Measured: Dockerfile.variant was 42251 B at v1.9.4 (base64 56355,
-          # fine) and 50034 B at v1.10.1 (base64 66712, truncated to 65536) —
-          # the cap corresponds to a 49152 B Dockerfile, so the v1.10.0/v1.10.1
-          # comment growth crossed it by 882 B. v1.10.0 run 704 and v1.10.1
-          # run 707 both died here; in 704 it hid behind a stale clipboard
-          # assertion. Trimming comments would "fix" it until the next comment.
-          # These smoke images are built with load: true and thrown away, so the
-          # attestation has no consumer. The PUBLISHED images are built by raw
-          # `docker buildx build --push` in run: blocks, which never writes
-          # $GITHUB_OUTPUT metadata — so this changes nothing about what ships.
-          provenance: false
           tags: pi-devbox:smoke
           build-args: |
             BASE_IMAGE=${{ env.IMAGE }}:${{ needs.base-decide.outputs.base_tag }}
@@ -680,28 +665,6 @@ jobs:
           platforms: linux/amd64
           push: false
           load: true
-          # provenance: false is LOAD-BEARING, not hygiene. buildx writes a
-          # provenance attestation into the metadata it hands back, and that
-          # metadata embeds the ENTIRE Dockerfile as one base64 "data" field on a
-          # single line. build-push-action writes the metadata to $GITHUB_OUTPUT
-          # as a `name<<ghadelimiter_<uuid>` heredoc, and Gitea's act_runner
-          # truncates any single line at exactly 65536 chars — so once
-          # base64(Dockerfile.variant) crosses 64 KiB the closing delimiter is
-          # cut off and the runner fails the job with
-          #   invalid format delimiter 'ghadelimiter_...' not found before end of file
-          # and NO failing step: every step reports Success, the smoke suite
-          # reports "0 failed", and the job is red anyway.
-          # Measured: Dockerfile.variant was 42251 B at v1.9.4 (base64 56355,
-          # fine) and 50034 B at v1.10.1 (base64 66712, truncated to 65536) —
-          # the cap corresponds to a 49152 B Dockerfile, so the v1.10.0/v1.10.1
-          # comment growth crossed it by 882 B. v1.10.0 run 704 and v1.10.1
-          # run 707 both died here; in 704 it hid behind a stale clipboard
-          # assertion. Trimming comments would "fix" it until the next comment.
-          # These smoke images are built with load: true and thrown away, so the
-          # attestation has no consumer. The PUBLISHED images are built by raw
-          # `docker buildx build --push` in run: blocks, which never writes
-          # $GITHUB_OUTPUT metadata — so this changes nothing about what ships.
-          provenance: false
           tags: pi-devbox:smoke-studio
           build-args: |
             BASE_IMAGE=${{ env.IMAGE }}:${{ needs.base-decide.outputs.base_tag }}