add support for COPY to docker build

This adds a COPY command to docker build which works like ADD, but is
only for local files and it doesn't extract files.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)

integration-cli/build_tests/TestCopy/SingleFileToRoot/Dockerfile

@@ -0,0 +1,9 @@
+FROM busybox
+RUN echo 'dockerio:x:1001:1001::/bin:/bin/false' >> /etc/passwd
+RUN echo 'dockerio:x:1001:' >> /etc/group
+RUN touch /exists
+RUN chown dockerio.dockerio /exists
+COPY test_file /
+RUN [ $(ls -l /test_file | awk '{print $3":"$4}') = 'root:root' ]
+RUN [ $(ls -l /test_file | awk '{print $1}') = '-rw-r--r--' ]
+RUN [ $(ls -l /exists | awk '{print $3":"$4}') = 'dockerio:dockerio' ]