feat(arm): Cross build arm and arm64 docker images

- Requires `qemu-user-static`, added to travis - maybe incorrect?
- Add build guide
- `.gitignore` `release/` directory

Makefile

@@ -45,6 +45,34 @@ build: clean $(BINARY)
 $(BINARY):
 	CGO_ENABLED=0 $(GO) build -a -installsuffix cgo -ldflags="-X main.VERSION=${VERSION}" -o $@ github.com/pusher/oauth2_proxy
 
+# Ubuntu 18.04.1 Qemu version
+QEMU-STATIC-VERSION=v2.11.1
+
+.PHONY: qemu-static
+qemu-static:
+	wget -O dist/qemu-amd64-static https://github.com/multiarch/qemu-user-static/releases/download/${QEMU-STATIC-VERSION}/qemu-x86_64-static
+	wget -O dist/qemu-aarch64-static https://github.com/multiarch/qemu-user-static/releases/download/${QEMU-STATIC-VERSION}/qemu-aarch64-static
+	wget -O dist/qemu-arm-static https://github.com/multiarch/qemu-user-static/releases/download/${QEMU-STATIC-VERSION}/qemu-arm-static
+	chmod +x dist/qemu-*-static
+
+.PHONY: qemu-register
+qemu-register:
+	docker run --rm --privileged multiarch/qemu-user-static:register
+
+.PHONY: docker
+docker:
+	docker build -f Dockerfile -t pusher/oauth2_proxy:latest .
+
+.PHONY: docker-all
+docker-all: qemu-static docker
+	docker build -f Dockerfile -t pusher/oauth2_proxy:latest-amd64 .
+	docker build -f Dockerfile -t pusher/oauth2_proxy:${VERSION} .
+	docker build -f Dockerfile -t pusher/oauth2_proxy:${VERSION}-amd64 .
+	docker build -f Dockerfile.arm64 -t pusher/oauth2_proxy:latest-arm64 .
+	docker build -f Dockerfile.arm64 -t pusher/oauth2_proxy:${VERSION}-arm64 .
+	docker build -f Dockerfile.armv6 -t pusher/oauth2_proxy:latest-armv6 .
+	docker build -f Dockerfile.armv6 -t pusher/oauth2_proxy:${VERSION}-armv6 .
+
 .PHONY: test
 test: dep lint
 	$(GO) test -v -race $(go list ./... | grep -v /vendor/)