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.md

@@ -167,7 +167,7 @@ In `memcached.sh` (make sure this file is chmod +x):
 In `Dockerfile`:
 
     RUN mkdir /etc/service/memcached
-    ADD memcached.sh /etc/service/memcached/run
+    COPY memcached.sh /etc/service/memcached/run
 
 Note that the shell script must run the daemon **without letting it daemonize/fork it**. Usually, daemons provide a command line flag or a config file option for that.
 
@@ -191,7 +191,7 @@ In `logtime.sh` (make sure this file is chmod +x):
 In `Dockerfile`:
 
     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>
 ### Environment variables
@@ -456,7 +456,7 @@ Instructions for logging in the container is the same as in section [Using the i
 Edit your Dockerfile to install an SSH public key:
 
     ## Install an SSH of your choice.
-    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
 
 Then rebuild your image. Once you have that, start a container based on that image: