ci/lib-docker: preserve required environment variables

We're using "su -m" to preserve environment variables in the shell run
by "su". But, that options will be ignored while "-l" (aka "--login") is
specified in util-linux and busybox's su.

In a later patch this script will be reused for checking Git for Linux
with musl libc on Alpine Linux, Alpine Linux uses "su" from busybox.

Since we don't have interest in all environment variables,
pass only those necessary variables to the inner script.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/run-linux32-build.sh

@@ -51,10 +51,17 @@ else
 fi
 
 # Build and test
-linux32 --32bit i386 su -m -l $CI_USER -c '
+linux32 --32bit i386 su -m -l $CI_USER -c "
 	set -ex
+	export DEVELOPER='$DEVELOPER'
+	export DEFAULT_TEST_TARGET='$DEFAULT_TEST_TARGET'
+	export GIT_PROVE_OPTS='$GIT_PROVE_OPTS'
+	export GIT_TEST_OPTS='$GIT_TEST_OPTS'
+	export GIT_TEST_CLONE_2GB='$GIT_TEST_CLONE_2GB'
+	export MAKEFLAGS='$MAKEFLAGS'
+	export cache_dir='$cache_dir'
 	cd /usr/src/git
-	test -n "$cache_dir" && ln -s "$cache_dir/.prove" t/.prove
+	test -n '$cache_dir' && ln -s '$cache_dir/.prove' t/.prove
 	make
 	make test
-'
+"