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  | 30 MB      | 81.6 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 +1,29 @@
 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 --virtual .build_deps \
+    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 .build_deps && \
+    rm -Rf /var/cache/apk/* && \
+    rm -Rf $GOPATH

Makefile

@@ -33,12 +33,12 @@ test:
 ci-coveralls: tools deps
 	goveralls -service=travis-ci
 
+# Builds Relay on the current OS
 exe: clean-dev | $(BUILD_DIR)
 	CGO_ENABLED=0 govendor build -ldflags "$(LINK_VARS)" -o $(BUILD_DIR)/$(EXENAME)
 
-docker: export GOOS=linux
-docker: export GOARCH=amd64
-docker: clean exe do-docker-build
+docker:
+	docker build -t $(DOCKER_IMAGE) .
 
 clean: clean-dev
 	rm -rf $(BUILD_DIR) relay-test
@@ -73,9 +73,3 @@ $(TARBALL_NAME): test exe
 	cp example_relay.conf $(TARBALL_NAME)
 	tar czf $(TARBALL_NAME).tar.gz $(TARBALL_NAME)
 	rm -rf $(TARBALL_NAME)
-
-# Providing this solely for CI-built images. We will have already
-# built the executable in a separate step. We split things up because
-# we build inside a Docker image in CI (we don't have Go on builders).
-do-docker-build:
-	docker build -t $(DOCKER_IMAGE) .