Enable multi-architecture docker image builds

Previously, the Dockerfile downloaded a 'docker-gen' binary during build
time. This caused a problem as it hard-coded the amd64 architecture for
the image.

This commit updates the Dockerfile to build the docker-gen executable
from source instead of downloading a binary. This updated Dockerfile
uses a multi-stage build [1]. The first stage downloads a source tarball
from github and produces a binary out of it. The second stage
corresponds to the old Dockerfile, with the sole change being that is
fetches the binary from the first stage instead of GitHub.

This has the advantage that the build process no longer assumes a
certain architecture and enables building the image for any architecture
that both the golang and alpine base images support.

[1] https://docs.docker.com/develop/develop-images/multistage-build/

Dockerfile

@@ -1,12 +1,29 @@
+# Build docker-gen from scratch
+FROM golang:1.14-alpine as dockergen
+RUN apk add --no-cache git
+
+# Download the sources for the given version
+ENV VERSION 0.7.3
+ADD https://github.com/jwilder/docker-gen/archive/${VERSION}.tar.gz sources.tar.gz
+
+# Move the sources into the right directory
+RUN tar -xzf sources.tar.gz && \
+   mkdir -p /go/src/github.com/jwilder/ && \
+   mv docker-gen-* /go/src/github.com/jwilder/docker-gen
+
+# Install the dependencies and make the docker-gen executable
+WORKDIR /go/src/github.com/jwilder/docker-gen
+RUN go get -v ./... && \
+   CGO_ENABLED=0 GOOS=linux go build -ldflags "-X main.buildVersion=${VERSION}" ./cmd/docker-gen
+
 FROM alpine:latest
 LABEL maintainer="Jason Wilder <mail@jasonwilder.com>"
 
 RUN apk -U add openssl
 
 ENV VERSION 0.7.3
-ENV DOWNLOAD_URL https://github.com/jwilder/docker-gen/releases/download/$VERSION/docker-gen-alpine-linux-amd64-$VERSION.tar.gz
+COPY --from=dockergen /go/src/github.com/jwilder/docker-gen/docker-gen /usr/local/bin/docker-gen
 ENV DOCKER_HOST unix:///tmp/docker.sock
 
-RUN wget -qO- $DOWNLOAD_URL | tar xvz -C /usr/local/bin
 
 ENTRYPOINT ["/usr/local/bin/docker-gen"]