fix(test): hand E2E evidence back to the right user under rootless Docker

The Linux E2E passes under rootless Docker, but its evidence came back
unreadable, and the run ended on sha256sum failing to open the manifest it had
just written.

The container writes evidence as its own non-root user. restore_evidence_owner
chowned that back to $(id -u) from inside the container, which is correct only
because rootful Docker maps uid to uid — the mapping CI runs under, which is
why this was never noticed. Rootless inverts it: container root maps to the
invoking user, and every other container id maps to a subuid the user cannot
read. Chowning to 1004 there produces another subuid, not the caller.

Detect the daemon mode and chown to 0:0 when rootless. Rootful behaviour, and
therefore CI, is unchanged.

Verified locally on rootless: evidence lands owned by the invoking user with
0600 intact, sha256sum -c passes, and the suite's own mode assertions still
hold. CONTRIBUTING notes that rootless is supported.

Author: yashranaway

CONTRIBUTING.md

@@ -43,6 +43,10 @@ Platform notes:
 - **Linux** builds need only Docker: `pnpm build:linux` compiles inside
   `swift:6.1-bookworm` and emits a tarball. Chromium must be non-Snap —
   the runtime rejects Snap launchers before starting, by design.
+- **Rootless Docker works**, including the E2E: Chromium's nested namespace
+  sandbox runs fine, and the harness detects the daemon mode so exported QA
+  evidence comes back owned by you. Nothing binds a host port, so the suites
+  cannot collide with other services on a shared machine.
 - `pnpm test:e2e:mac` opens real windows and mutates `com.headless.app` user
   defaults. Don't run it in a background session or on a machine where that
   matters.

apps/headless/Tests/linux-docker.sh

@@ -8,9 +8,19 @@ EVIDENCE_ROOT="${HEADLESS_EVIDENCE_ROOT:-$PWD/build/qa-evidence}"
 mkdir -p "$EVIDENCE_ROOT"
 EVIDENCE_DIR="$(mktemp -d "$EVIDENCE_ROOT/linux.XXXXXX")"
 chmod 0777 "$EVIDENCE_DIR"
+# The container writes evidence as its own non-root user, which lands on the
+# host as an id this user cannot read. Which id to hand it back to depends on
+# how the daemon maps them: rootful Docker maps uid to uid, so the invoking
+# user's id is correct, while rootless maps container root to the invoking user
+# and every other container id to an unreadable subuid.
+if docker info --format '{{join .SecurityOptions ","}}' 2>/dev/null | grep -q rootless; then
+  EVIDENCE_OWNER="0:0"
+else
+  EVIDENCE_OWNER="$(id -u):$(id -g)"
+fi
 restore_evidence_owner() {
   docker run --rm --user root -v "$EVIDENCE_DIR:/evidence" \
-    headless-p1-test chown -R "$(id -u):$(id -g)" /evidence >/dev/null 2>&1 || true
+    headless-p1-test chown -R "$EVIDENCE_OWNER" /evidence >/dev/null 2>&1 || true
   chmod 0700 "$EVIDENCE_DIR" >/dev/null 2>&1 || true
 }
 file_mode() {