[skip ci] Update documentation of ci/Dockerfile

ci/Dockerfile

@@ -1,5 +1,6 @@
-# Dockerfile to build nutcracker
-# ARGS: none
+# Dockerfile to create a slower debug build of nutcracker with assertions enabled
+# for continuous integration checks.
+# ARGS: REDIS_VER
 # Also see test_in_docker.sh
 FROM centos:7
 
@@ -37,8 +38,8 @@ RUN wget https://github.com/redis/redis/archive/$REDIS_VER.tar.gz && \
 	popd && \
 	rm -r redis-*
 
-# This will build and twemproxy 0.4.1 (with sflow)
-# Annoyingly, can't add multiple directories at once.
+# This will build twemproxy with compilation flags for running unit tests, integration tests.
+# Annoyingly, this can't add multiple directories at once.
 ADD conf /usr/src/twemproxy/conf
 ADD contrib /usr/src/twemproxy/contrib
 ADD man /usr/src/twemproxy/man