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).

image/Dockerfile

@@ -1,7 +1,7 @@
 FROM ubuntu:14.04
 MAINTAINER Phusion <info@phusion.nl>
 
-ADD . /bd_build
+COPY . /bd_build
 
 RUN /bd_build/prepare.sh && \
 	/bd_build/system_services.sh && \