bake: do not leak target overrides into inherited entries

[breken-ai]

Targets that inherit from the same parent share the parent's output and secret entries, because Target.Merge copies the slices and pointers as they are. Two --set overrides change those entries in place, so an override for one target also changes every other target that inherits the same entries:

• setPushOverride writes Attrs["push"] on the shared ExportEntry, and for push=false it swaps entries inside the shared slice.
• updateSecret (the secret.<id>= source override) writes Env and FilePath on the shared Secret.

target "_common" {
  output = ["type=image"]
  secret = ["id=token,env=COMMON_TOKEN"]
}
target "api" {
  inherits = ["_common"]
  tags = ["example.com/api"]
}
target "app" {
  inherits = ["_common"]
  tags = ["example.com/app"]
}

docker buildx bake --print --set app.push=true --set app.secret.token=env=APP_TOKEN api app on master:

api output=[{push: true, type: image}] secret=[{id: token, env: APP_TOKEN}]
app output=[{push: true, type: image}] secret=[{id: token, env: APP_TOKEN}]

So --set app.push=true also pushes api, and api gets app's secret source. With this change:

api output=[{type: image}]             secret=[{id: token, env: COMMON_TOKEN}]
app output=[{push: true, type: image}] secret=[{id: token, env: APP_TOKEN}]

Fix: copy before changing. setPushOverride now builds a new slice and clones any entry whose push attribute it sets (withExportAttr). updateSecret replaces the entry in a cloned slice. For push=false the output order is kept now instead of being swapped.

Tests

• New TestPushOverride/inherited_outputs (app.push=true and app.push=false) and TestSecretSourceOverrideInherited. On master they fail because api gets push=true and env=APP_TOKEN, and they pass with the fix.
• go test ./bake/... passes. golangci-lint v2.8.0 (the version in hack/dockerfiles/lint.Dockerfile) reports 0 issues on ./bake/....
• End to end: the --print output above is from binaries built from master and from this branch.

AI disclosure: I found and fixed this with help from an AI coding assistant (Claude). I reviewed the change and ran the tests above. The commit has an Assisted-By: trailer and a DCO sign-off.