Fix get.docker.com not working for RCs

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>

Dockerfile.osx

@@ -29,11 +29,15 @@ RUN curl -fsSL -o /vbox.dmg http://download.virtualbox.org/virtualbox/$VBOX_VERS
 ENV INSTALLER_VERSION 1.12.2-rc1
 ENV DOCKER_VERSION 1.12.2-rc1
 
+# Change this to 'test.docker.com' for RCs and 'get.docker.com' for official
+# releases.
+ENV DOCKER_ENDPOINT test.docker.com
+
 # Why '--strip-components 3'?  The client binary is in usr/local/bin/docker in
 # the tar archive.  If we extract directly, it will create a usr/local/bin
 # subdirectory (not what we want).  So we use --strip-components to remove the
 # `usr/local/bin` part and drop the bin in the current directory.
-RUN curl -fsSL -o dockerbins.tgz "https://get.docker.com/builds/Darwin/x86_64/docker-${DOCKER_VERSION}.tgz" && \
+RUN curl -fsSL -o dockerbins.tgz "https://${DOCKER_ENDPOINT}/builds/Darwin/x86_64/docker-${DOCKER_VERSION}.tgz" && \
     tar xvf dockerbins.tgz docker/docker --strip-components 1 && \
     rm dockerbins.tgz
 

Dockerfile.windows

@@ -32,7 +32,11 @@ RUN mkdir /bundle
 
 WORKDIR /bundle
 
-RUN curl -fsSL -o dockerbins.zip "https://get.docker.com/builds/Windows/x86_64/docker-${DOCKER_VERSION}.zip" && \
+# Change this to 'test.docker.com' for RCs and 'get.docker.com' for official
+# releases.
+ENV DOCKER_ENDPOINT test.docker.com
+
+RUN curl -fsSL -o dockerbins.zip "https://${DOCKER_ENDPOINT}/builds/Windows/x86_64/docker-${DOCKER_VERSION}.zip" && \
     unzip dockerbins.zip && \
     mv docker/docker.exe . && \
     rm -r docker/ dockerbins.zip