allow building as root inside a container without complaint

add a check for $container

Change-Id: Ib6921c6d771622fb5f4acb82d10aa6fb34e1bbac
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/96538
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>

Makefile.in

@@ -59,7 +59,7 @@ else # MAKE_RESTARTS
 .DEFAULT_GOAL := build
 
 check-if-root:
-	@if test ! `uname` = 'Haiku' -a `id -u` = 0 && ! grep -q 'lxc\|docker' /proc/self/cgroup && ! grep -q 'libpod_parent' /proc/self/cgroup; then \
+	@if test ! `uname` = 'Haiku' -a `id -u` = 0 && test -z $$container && ! grep -q 'lxc\|docker' /proc/self/cgroup && ! grep -q 'libpod_parent' /proc/self/cgroup; then \
 		echo; \
 		echo 'Building LibreOffice as root is a very bad idea, use a regular user.'; \
 		echo; \