Use COPY instead of ADD

While COPY and ADD are essentially interchangeable here, it is still
considered good practice to use COPY whenever possible. From the docker
docks on best practices:

"Although ADD and COPY are functionally similar, generally speaking,
COPY is preferred. That’s because it’s more transparent than ADD.
[...] For other items (files, directories) that do not require ADD’s tar
auto-extraction capability, you should always use COPY."

Additionally, ADD commands were not cached prior to 0.7.3 (which was
released on 2013-01-02).

README_zh_tw.md

@@ -156,7 +156,7 @@ The image is called `phusion/baseimage`, and is available on the Docker registry
 	
 	### 在Dockerfile中:
     RUN mkdir /etc/service/memcached
-    ADD memcached.sh /etc/service/memcached/run
+    COPY memcached.sh /etc/service/memcached/run
 
 注意腳本必須運行在後臺的,**不能讓他們行程進行daemonize/fork**.通常,後臺行程會提供一個標誌位或者配置文件.
 
@@ -178,7 +178,7 @@ baseimage-docker的初始化腳本 `/sbin/my_init`,在啓動的時候行程運
 
     ### 在 Dockerfile中:
     RUN mkdir -p /etc/my_init.d
-    ADD logtime.sh /etc/my_init.d/logtime.sh
+    COPY logtime.sh /etc/my_init.d/logtime.sh
 
 
 <a name="environment_variables"></a>
@@ -487,7 +487,7 @@ Baseimage-docker提供了一個靈活的方式運行只要一閃而過的命令,
 編輯你的dockerfile,來安裝ssh public key:
 
     ## 安裝你自己的public key.
-    ADD your_key.pub /tmp/your_key.pub
+    COPY your_key.pub /tmp/your_key.pub
     RUN cat /tmp/your_key.pub >> /root/.ssh/authorized_keys && rm -f /tmp/your_key.pub
 
 重新創建你的鏡像.一旦你創建成功,啓動基於這個鏡像的容器.