reorder Dockerfile steps for better layer caching

Running `apk add` before copying source into the image takes better
adavantage of layer caching when developing and regularly building the
image. This avoids source code changes invalidating the `apk add` layer
and causing that step to run on every image build.

Signed-off-by: Adam Duke <adam.v.duke@gmail.com>

Dockerfile

@@ -3,13 +3,13 @@ FROM golang:1.6-alpine
 ENV DISTRIBUTION_DIR /go/src/github.com/docker/distribution
 ENV DOCKER_BUILDTAGS include_oss include_gcs
 
+RUN set -ex \
+    && apk add --no-cache make git
+
 WORKDIR $DISTRIBUTION_DIR
 COPY . $DISTRIBUTION_DIR
 COPY cmd/registry/config-dev.yml /etc/docker/registry/config.yml
 
-RUN set -ex \
-    && apk add --no-cache make git
-
 RUN make PREFIX=/go clean binaries
 
 VOLUME ["/var/lib/registry"]