Docker: ship a static binary with alpine image

Update Makefile and Dockerfile to build a static binary by default and
get a much smaller footprint with alpine image.  It also includes CA
certificates needed for practical use.

Optimize ordering of build steps in Dockerfile to avoid needless
downloads.

Add Dockerfile.dev in .gitignore and .dockerignore.  It's to utilize
cached build steps on Dockerfile development.

.dockerignore

@@ -0,0 +1 @@
+Dockerfile.dev

.gitignore

@@ -29,3 +29,10 @@ _testmain.go
 
 # Editor swap/temp files
 .*.swp
+
+# Dockerfile.dev is ignored by both git and docker
+# for faster development cycle of docker build
+# cp Dockerfile Dockerfile.dev
+# vi Dockerfile.dev
+# docker build -f Dockerfile.dev .
+Dockerfile.dev

Dockerfile

@@ -1,18 +1,22 @@
 FROM golang:1.11-stretch AS builder
-WORKDIR /go/src/github.com/pusher/oauth2_proxy
+
+# Download tools
+RUN wget -O $GOPATH/bin/dep https://github.com/golang/dep/releases/download/v0.5.0/dep-linux-amd64
+RUN chmod +x $GOPATH/bin/dep
+
+# Copy sources
+WORKDIR $GOPATH/src/github.com/pusher/oauth2_proxy
 COPY . .
 
 # Fetch dependencies
-RUN wget -O dep https://github.com/golang/dep/releases/download/v0.5.0/dep-linux-amd64
-RUN chmod +x dep
-RUN mv dep $GOPATH/bin/dep
 RUN dep ensure --vendor-only
 
-# Build image
-RUN ./configure && make clean oauth2_proxy
+# Build binary
+RUN ./configure && make build
 
-# Copy binary to debian
-FROM debian:stretch
+# Copy binary to alpine
+FROM alpine:3.8
+RUN apk add --no-cache ca-certificates
 COPY --from=builder /go/src/github.com/pusher/oauth2_proxy/oauth2_proxy /bin/oauth2_proxy
 
 ENTRYPOINT ["/bin/oauth2_proxy"]

Makefile

@@ -43,7 +43,7 @@ dep:
 build: clean $(BINARY)
 
 $(BINARY):
-	$(GO) build -ldflags="-X main.VERSION=${VERSION}" -o $(BINARY) github.com/pusher/oauth2_proxy
+	CGO_ENABLED=0 $(GO) build -a -installsuffix cgo -ldflags="-X main.VERSION=${VERSION}" -o $@ github.com/pusher/oauth2_proxy
 
 .PHONY: test
 test: dep lint