fix(apparmor): keep docker-default loaded when tearing AppArmor down

`security-apparmor-teardown` ran `aa-teardown`, which unloads every
profile including dockerd's in-memory `docker-default`. dockerd only loads
that profile when it starts, so every container start afterwards that
requests it fails until dockerd restarts. Since #1067 runs the teardown on
the GitHub runners, BuildKit failed each RUN of the swarm node image three
times (run 39332343490, 39 of 40 swarm jobs):

    runc run failed: unable to start container process: error during
    container init: unable to apply apparmor profile: apparmor failed to
    apply profile: write .../attr/apparmor/exec: no such file or directory

The teardown now removes each loaded profile by name through securityfs
and skips `docker-default` and child profiles, which leave with their
parent. Dev hosts gain the same: no dockerd restart is needed after
`make environment-bootstrap`.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/system/apparmor/common.sh

@@ -43,6 +43,10 @@ apparmor_should_manage() {
 	apparmor_is_installed && apparmor_is_enabled
 }
 
+apparmor_removable_profiles() {
+	sed -E 's/ \([a-z]+\)$//' | grep -v -E '//|^docker-default$'
+}
+
 systemd_is_operational() {
 	command -v systemctl >/dev/null 2>&1 && [[ -d /run/systemd/system ]]
 }

scripts/system/apparmor/teardown.sh

@@ -18,12 +18,15 @@ else
 	echo "[apparmor] apparmor.service not available or systemd inactive; skipping service stop"
 fi
 
-if command -v aa-teardown >/dev/null 2>&1; then
-	aa-teardown || apparmor_warn "[apparmor] aa-teardown returned non-zero; continuing"
-elif command -v apparmor_parser >/dev/null 2>&1 && compgen -G '/etc/apparmor.d/*' >/dev/null; then
-	apparmor_parser -R /etc/apparmor.d/* || true # nocheck: shell-or-true -- grandfathered: worked in practice; TODO: sharpen to catch only the exact tolerated error
+APPARMOR_FS=/sys/kernel/security/apparmor
+if [[ -w "${APPARMOR_FS}/.remove" ]]; then
+	mapfile -t profiles < <(apparmor_removable_profiles <"${APPARMOR_FS}/profiles")
+	for profile in "${profiles[@]}"; do
+		printf '%s' "${profile}" >"${APPARMOR_FS}/.remove"
+	done
+	echo "[apparmor] unloaded ${#profiles[@]} profile(s); docker-default stays loaded"
 else
-	echo "[apparmor] no AppArmor profile tooling available; skipping profile unload"
+	echo "[apparmor] ${APPARMOR_FS}/.remove is not writable; skipping profile unload"
 fi
 
 echo "[apparmor] teardown complete"

tests/unit/python/scripts/system/apparmor/test_common.py

@@ -0,0 +1,48 @@
+"""The AppArmor teardown must keep docker-default loaded.
+
+Run 39332343490 lost 39 swarm jobs: the runner's teardown unloaded every
+profile, and BuildKit then failed each node image RUN step with
+`apparmor failed to apply profile: write .../attr/apparmor/exec: no such file`.
+"""
+
+from __future__ import annotations
+
+import subprocess
+import unittest
+
+from utils.cache.files import PROJECT_ROOT
+
+COMMON = PROJECT_ROOT / "scripts" / "system" / "apparmor" / "common.sh"
+
+PROFILES = """wg-quick (enforce)
+wg-quick//ip (enforce)
+docker-default (enforce)
+/usr/bin/man (enforce)
+unprivileged_userns (unconfined)
+docker-defaults (complain)
+"""
+
+
+class TestRemovableProfiles(unittest.TestCase):
+    def test_keeps_docker_default_and_child_profiles(self) -> None:
+        result = subprocess.run(
+            ["bash", "-c", f'source "{COMMON}"; apparmor_removable_profiles'],
+            input=PROFILES,
+            capture_output=True,
+            text=True,
+            check=True,
+        )
+
+        self.assertEqual(
+            [
+                "wg-quick",
+                "/usr/bin/man",
+                "unprivileged_userns",
+                "docker-defaults",
+            ],
+            result.stdout.splitlines(),
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()