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isArchive() decides whether stdin carries a build context or a raw
Dockerfile by sniffing magic bytes. The table knows bzip2, gzip, xz and
plain tar, but not zstd, so a zstd-compressed context piped to
'docker build -' is parsed as a Dockerfile and the build fails on line 1:
$ echo 'FROM scratch' > Dockerfile
$ tar cf ctx.tar Dockerfile && zstd -q ctx.tar -o ctx.tar.zst
$ docker build --check - < ctx.tar.zst
1 | >>> (\u2572/\u2572d '5 \u2572F`i...
gzip and plain tar on the same path work.
The daemon already decompresses zstd build contexts - verified against
Docker Engine 29.7.2 by building the same tar.zst context through the
/build API directly - so only the client-side sniff rejects it today.
This adds the zstd frame magic to the table and a unit test covering
all supported formats.
Related: moby/go-archive#13
Signed-off-by: nrvate <11264848+nrvate@users.noreply.github.com>
crazy-max
requested changes
Sep 15, 2026
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I initially thought moby/buildkit#7069 covered this, but Buildx needs to recognize the archive before uploading it, so this fix is needed.
I’ve extracted a shared compression.IsArchive helper from moby/buildkit#7034 in moby/buildkit#7148, including zstd skippable-frame support. Once that lands and we vendor it here, we can replace Buildx local detector with the shared helper and cover zstd contexts with a stdin integration test. That should keep the two implementations from drifting again.
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Problem
docker build -decides whether stdin carries a build context or a raw Dockerfile by sniffing magic bytes (isArchiveinbuild/utils.go). The table knows bzip2, gzip, xz and plain tar — but not zstd. A zstd-compressed context on stdin is therefore parsed as a Dockerfile and the build fails immediately:gzip and plain tar on the same path work.
Fix
Add the zstd frame magic (
0x28, 0xB5, 0x2F, 0xFD) to the table. Nothing else changes: for the default docker driver the stream is passed through to the daemon, and the daemon already decompresses zstd build contexts — verified against Docker Engine 29.7.2 by building the same tar.zst context through the/buildAPI directly (it succeeds today; only this client-side sniff rejects it).Testing
TestIsArchivecovering plain tar, gzip, bzip2, xz, zstd, a Dockerfile-from-stdin negative, and a truncated-magic negativego test ./build/ -run TestIsArchivepasses (Go 1.26, per go.mod)Related: moby/go-archive#13 (broader zstd support in the archive library — this PR is the narrower client-side sniff gap).