Use native buildkit symlink file action for post-install symlinks

[cpuguy83]

What this PR does / why we need it:

Post-install symlinks (image.post.symlinks) were created by executing a shell
script (ln -s, chown -h, chgrp -h) in the worker, mounting /etc/passwd
and /etc/group to resolve ownership by name. Buildkit now provides a native
symlink file action, so this switches to it when it is available.

Support is detected via the pb.CapFileSymlinkCreate LLB capability, mirroring
the existing diff/merge capability detection. When the connected buildkit does
not advertise the capability — or DALEC_DISABLE_SYMLINK=1 is set — Dalec falls
back to the original shell-exec implementation, so older buildkit versions keep
working.

The native path chains llb.Mkdir (for parent directories, which the symlink
action does not create itself) and llb.Symlink into a single file op resolved
against the installed rootfs. This avoids the extra exec and the /etc/passwd
/ /etc/group mounts.

Ownership semantics are preserved exactly. Chowning by user name alone would
also set the group to that user's primary group, whereas the exec fallback
leaves the unspecified side as root. To match, both user and group are always
set explicitly and an unspecified side stays 0, mirroring chown -h user /
chgrp -h group.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #

Special notes for your reviewer:

• Capability detection lives in frontend/gateway.go (SupportsSymlink) and is
  wired in frontend/router.go, just like DisableDiffMerge.
• DALEC_DISABLE_SYMLINK is registered as a known build-arg in load.go and can
  be used to force the legacy behavior.
• Unit tests in helpers_symlink_test.go assert the marshaled LLB for both the
  native and exec paths (symlink actions, parent mkdirs, and ownership).
• The integration test testLinuxPostInstallSymlinks runs the post-install
  symlink scenario under both paths via subtests — native by default and the
  exec fallback with DALEC_DISABLE_SYMLINK=1. The equivalent block was removed
  from the large combined container test to avoid duplicating the coverage.

[Copilot]

Pull request overview

This PR switches the post-install symlink creation (image.post.symlinks) from executing a shell script (ln -s, chown -h, chgrp -h) in a worker container to using BuildKit's native llb.Symlink file action when the connected BuildKit instance supports it (via pb.CapFileSymlinkCreate). When the capability is unavailable or DALEC_DISABLE_SYMLINK=1 is set, the original exec-based implementation is used as a fallback.

Changes:

• Added capability detection (SupportsSymlink) in frontend/gateway.go and wired it in frontend/router.go, following the established DisableDiffMerge/SupportsDiffMerge pattern. The DALEC_DISABLE_SYMLINK build arg is registered in load.go.
• Refactored InstallPostSymlinks in helpers.go into two internal functions—installPostSymlinksNative (BuildKit file action chain with llb.Mkdir + llb.Symlink) and installPostSymlinksExec (preserved shell-exec fallback)—with ownership semantics that explicitly set unspecified user/group to root (id 0).
• Extracted post-install symlink integration tests from the combined container test into a dedicated testLinuxPostInstallSymlinks function that runs both native and exec fallback paths, and added comprehensive unit tests in helpers_symlink_test.go asserting the marshaled LLB output.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
frontend/gateway.go	Added SupportsSymlink/checkSymlink for CapFileSymlinkCreate detection, mirroring SupportsDiffMerge
frontend/router.go	Wired DisableSymlink(true) when symlink capability is not supported
helpers.go	Split InstallPostSymlinks into native (llb.Symlink file action) and exec (ln -s shell script) paths; added symlinkChown helper
load.go	Registered DALEC_DISABLE_SYMLINK as a known build arg
helpers_symlink_test.go	New unit tests verifying marshaled LLB ops for both native and exec paths
test/linux_target_test.go	Moved symlink tests to dedicated testLinuxPostInstallSymlinks with subtests for native and exec-fallback paths

[invidian]

@cpuguy83 can you resolve conflicts?

[invidian]

Should we also add some documentation around this feature environment variable?

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[cpuguy83]

Should we also add some documentation around this feature environment variable?

The args are not so much a feature as they are a way for us to test both modes. I'd rather not document them for public use.

[invidian]

Just some nitpicks, overall LGTM