Add a test for docker exec

Author: YungSang

Makefile

@@ -38,6 +38,8 @@ test: test/Vagrantfile boot2docker-virtualbox.box
 	nc localhost 8080; \
 	echo '-----> docker-enter `docker ps -l -q` ls -l'; \
 	vagrant ssh -c 'docker-enter `docker ps -l -q` ls -l'; \
+	echo '-----> docker exec `docker ps -l -q` ls -l'; \
+	docker exec `docker ps -l -q` ls -l; \
 	vagrant suspend
 
 ptest: DOCKER_HOST_IP=$(shell cd test; vagrant ssh-config | sed -n "s/[ ]*HostName[ ]*//gp")
@@ -58,6 +60,8 @@ ptest: ptestup
 	nc ${DOCKER_HOST_IP} 8080; \
 	echo '-----> docker-enter `docker ps -l -q` ls -l'; \
 	vagrant ssh -c 'docker-enter `docker ps -l -q` ls -l'; \
+	echo '-----> docker exec `docker ps -l -q` ls -l'; \
+	docker exec `docker ps -l -q` ls -l; \
 	vagrant suspend
 
 ptestup: test/Vagrantfile boot2docker-parallels.box