Move docker-builds to OSDC runners

[huydhn]

Part 1 of moving ExecuTorch's Linux CI to OSDC. #22107 migrates linux_job_v2 -> v3, #22108 the native jobs.

OSDC pods have no docker daemon, so the builds move to the in-cluster BuildKit pool: build.sh switches to docker buildx build --push under REMOTE_BUILDKIT, driven by test-infra's docker-build-remote-buildkit action. The tag is computed in the workflow now, but it is the same string calculate-docker-image produced, so jobs still on v2 keep resolving.

Also adds the ciflow/docker trigger, registers it in pytorch-probot.yml and auto-labels the paths that need a rebuild: a fork PR gets no OIDC token, so a pull_request-triggered build cannot push on OSDC. pull_request stays for now.

Authored with Claude Code.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22106

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❌ 1 New Failure, 167 Pending

As of commit 5723f87 with merge base 9a2d135 ():

NEW FAILURE - The following job has failed:

• Build Presets / apple (macos) / build (gh)
  ##[error]fatal: unable to access 'https://github.com/Maratyszcza/FP16.git/': Failed to connect to github.com port 443 after 231 ms: Couldn't connect to server

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[huydhn]

Correcting the arm64 label. mt-l-arm64g2-6-25 does not schedule — the three arm64 legs of the first run sat queued from 19:09 to 22:56 and were cancelled without a runner ever picking them up.

The label naming is contradictory across sources: arc.yaml maps linux.arm64.2xlarge to l-arm64g2-6-32, ci-infra renamed that def to l-arm64g2-6-25 in pytorch/ci-infra#625, and ci-infra's smoke test still references c-mt-l-arm64g2-6-32 — but c-mt- is the staging prefix, so that says nothing about prod. The only recent runs on any mt-l-arm64g2-* label I could find anywhere are test-infra's build-clang/build-gcc from March.

So both PRs now use mt-l-arm64g4-16-62, which is what pytorch/pytorch's own OSDC docker-builds uses and which picks up in seconds in prod today. #22107 also folds in the one job that was on mt-l-arm64g3-16-62 — same size, equally unattested — so all arm64 work lands on one proven label.