New dockerfile that can build by itself

Previously, we'd build a glibc Relay locally and then stuff that into a
minimal Docker image and provide that. Now, we do the build of Relay
inside the image build itself. I have confirmed that an executable built
in this Alpine image runs fine in a Debian (i.e., glibc) system.

However, this results in a larger image size right now. Compare the
previous sizes to what is currently built by this commit:

| Image    | Compressed | Uncompressed |
| -------- | ---------- | ------------ |
| Previous | 6 MB       | 19.9 MB      |
| Current  | 31 MB      | 82.4 MB      |

This allows us to easily automate the build with DockerHub, though,
because it doesn't require anything besides the Dockerfile.

See operable/cog#1384 for more.

Dockerfile

@@ -1,10 +1,28 @@
 FROM alpine:3.4
 
-MAINTAINER Kevin Smith <[email protected]>
+MAINTAINER Christopher Maier <[email protected]>
 
-# Bake in a directory that we can use for logging, config, etc.
-RUN mkdir -p /var/operable/relay
+ENV GO_PACKAGE_VERSION 1.6.3-r0
+ENV GOPATH /gopath
+ENV PATH=${GOPATH}/bin:${PATH}
 
-# Relies on the binary having already been built
-COPY _build/relay /usr/local/bin
-COPY docker/relay.conf /usr/local/etc/relay.conf
+WORKDIR /gopath/src/github.com/operable/go-relay
+COPY . /gopath/src/github.com/operable/go-relay
+
+RUN apk -U add \
+    go=$GO_PACKAGE_VERSION \
+    go-tools=$GO_PACKAGE_VERSION \
+    git make && \
+
+    go get -u github.com/kardianos/govendor && \
+    make exe && \
+
+    mv _build/relay /usr/local/bin && \
+    mkdir -p /usr/local/etc && \
+    cp docker/relay.conf /usr/local/etc/relay.conf && \
+
+    # Provide a place to dump log files, etc.
+    mkdir -p /var/operable/relay && \
+
+    apk del go go-tools git make && \
+    rm -Rf $GOPATH