Makefile: fix "No such file or directory" error
$ make test-docker
grep: /sys/kernel/security/lsm: No such file or directory
CI_IMAGE_NAME = go-ceph-ci
CONTAINER_CMD ?=
-CONTAINER_OPTS := --security-opt $(shell grep -q selinux /sys/kernel/security/lsm && echo "label=disable" || echo "apparmor:unconfined")
+CONTAINER_OPTS := --security-opt $(shell grep -q selinux /sys/kernel/security/lsm 2>/dev/null && echo "label=disable" || echo "apparmor:unconfined")
CONTAINER_CONFIG_DIR := testing/containers/ceph
VOLUME_FLAGS :=
CEPH_VERSION := nautilus
