refactor(cd): drop the dedicated stage for the scratch image's /tmp

[defangdevs]

Follow-up to #474, answering @lionello's question there: no, that line is not needed.

build-base's own /tmp is empty (verified: ls -la /tmp in that stage shows nothing but . and ..), so it can be copied directly and the tmpdir stage goes away.

The -m 1777 was not doing anything either. Listing the exported image, /tmp comes out drwxr-xr-x root-owned in all three variants — the tmpdir stage as merged, a plain copy from build-base, and a copy with --chmod=1777. The CD runs as root, so the mode never mattered.

What does still matter is that the directory is copied rather than conjured by WORKDIR, which is builder-specific: BuildKit materialises it, buildah/podman does not, and the image then dies at deploy time with unable to create tmp directory for workspace: stat /tmp: no such file or directory. That part of #474 stays.

Verified by building the gcp target with podman and confirming tmp/ is present in the exported image, identical to what #474 produced.

Summary by CodeRabbit

• Chores
  • Simplified container image setup by streamlining temporary directory handling.
  • Preserved expected temporary file access and permissions in the final image.

[coderabbitai]

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📝 Walkthrough

Walkthrough

The Dockerfile removes the tmpdir stage and its /empty-tmp directory. The final cd-base image now copies the existing /tmp directory from build-base.

Changes

Docker image build

Layer / File(s)	Summary
Copy the existing temporary directory Dockerfile	The Dockerfile removes the dedicated tmpdir stage and copies /tmp from build-base into the final image.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 9cf7c

This change removes a redundant build stage while preserving the explicit /tmp directory needed for deployment portability; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the removal of the dedicated /tmp build stage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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