Fix environment-only Dockerfile builds #47
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wlan0
reviewed
Oct 30, 2025
pkg/build/frontend.go
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| // OCI manifests require a layers array (cannot be null) | ||
| stateWithLayer := llb.Scratch(). | ||
| File(llb.Mkfile("/.container-metadata-only", 0644, []byte("# This image contains only metadata (ENV/ARG/LABEL)\n"))). |
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Use a constant for marker path, that way it is self-documenting
const metadataOnlyMarker = "/.container-metadata-only"use that const instead of the literal
wlan0
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Oct 30, 2025
dcantah
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Oct 30, 2025
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jglogan
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Oct 30, 2025
- Fixes #736. - BuildKit returns nil ref for Dockerfiles containing only metadata (`ENV/ARG/LABEL`) directives without filesystem operations (`RUN/COPY/ADD`). Previously, this caused builds to fail with "no build directives" error. - Builder-shim 0.6.3 [fixes this](apple/container-builder-shim#47) by creating a minimal marker layer when ref is nil but image config is valid, satisfying OCI manifest requirements. - Also, added some tests for this behavior.
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When a Dockerfile only includes metadata directives like
ENV, ARG, or LABELand doesn’t perform any filesystem operations (RUN, COPY, ADD), BuildKit ends up returning a nil ref. Our builder currently treats that as an error, which blocks these metadata only Dockerfiles from building.This PR makes the builder handle that case gracefully by creating a small marker layer (
/.container-metadata-only) when ref is nil but the image is otherwise valid. That satisfies OCI manifest requirements and ensures the Swift decoder always gets a valid layers array instead of null, allowing metadata only Dockerfiles to build as expected.