Support extracting remote tarballs when used as named contexts with Bake

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I've found a bug and checked that ...

• ... the documentation does not mention anything about my problem
• ... there are no open or closed issues that are related to my problem

Description

When using a remote tarball (e.g. GitHub release archives) as a context or contexts entry in bake, the tarball is downloaded but not extracted.

Expected behaviour

Remote tarballs should be extracted and the contents should be used as a context. 1
Have Bake detect and extract tarball archives so that tarballs can be used as contexts directly. Similar to how build handles it:

• https://github.com/docker/buildx/blob/a6e198a341aff447882e30ce7ea3eb0b8f240391/build/opt.go#L399-L422

Actual behaviour

The tarball is downloaded but not extracted.

Buildx version

github.com/docker/buildx v0.26.1-desktop.1 532a478c2ea39e2d0eb40ad2e3f6bec57df4c8af

Docker info

Builders list

docker buildx ls
NAME/NODE           DRIVER/ENDPOINT     STATUS     BUILDKIT   PLATFORMS
container           docker-container                          
 \_ container0       \_ desktop-linux   inactive              
default             docker                                    
 \_ default          \_ default         running    v0.23.2    linux/amd64 (+2), linux/arm64, linux/ppc64le, linux/s390x, (2 more)
desktop-linux*      docker                                    
 \_ desktop-linux    \_ desktop-linux   running    v0.23.2    linux/amd64 (+2), linux/arm64, linux/ppc64le, linux/s390x, (2 more)

Configuration

target "default" {
  contexts = {
    buildx = "https://github.com/docker/buildx/archive/refs/tags/v0.27.0.tar.gz"
  }
  dockerfile-inline = <<EOT
FROM alpine:3.22
COPY --from=buildx . .
RUN ls -l && stop
EOT
}

Build logs

Additional info

• Note: GitHub release tarballs 302-redirect to a codeload.github.com URL.

• Even using the codeload URL directly still results in just the compressed tarball being available.

• It looks like Bake just sets the context state to the file rather than extracting it:

  • https://github.com/docker/buildx/blob/a6e198a341aff447882e30ce7ea3eb0b8f240391/bake/bake.go#L1291-L1292

  • https://github.com/docker/buildx/blob/a6e198a341aff447882e30ce7ea3eb0b8f240391/bake/bake.go#L1305-L1310

Start by comparing build/opt.go lines 399-422 with the Bake paths in bake/bake.go at lines 1291-1292 and 1305-1310. Reproduce the configuration using the remote tarball context, then follow how the context is assigned. Done means Bake extracts the remote tarball and makes its contents available to the build context, matching build behavior.