test/docker: check a continaer with a read-only file system

Now it's probably one valide use case, because there is no way to commit
a container when a container is being checkpointed.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/travis/docker-test.sh

@@ -40,7 +40,7 @@ docker info
 
 criu --version
 
-docker run --security-opt=seccomp:unconfined --name cr -d alpine /bin/sh -c 'i=0; while true; do echo $i; i=$(expr $i + 1); sleep 1; done'
+docker run --tmpfs /tmp --tmpfs /run --read-only --security-opt=seccomp:unconfined --name cr -d alpine /bin/sh -c 'i=0; while true; do echo $i; i=$(expr $i + 1); sleep 1; done'
 
 sleep 1
 for i in `seq 50`; do