Build a docker container

Use debian/distroless (glibc) as a base which in general gives
slightly higher performance than alpine (musl) while trading
off container size (about 10mb).

The build phase makes the assumption that any capnproto-files
are pre-processed before building in-docker.

Makefile

@@ -31,6 +31,10 @@ clean:
 cloudflared: tunnel-deps
 	go build -v $(VERSION_FLAGS) $(IMPORT_PATH)/cmd/cloudflared
 
+.PHONY: container
+container:
+	docker build -t cloudflare/cloudflared:"$(VERSION)" .
+
 .PHONY: test
 test: vet
 	go test -v -race $(VERSION_FLAGS) ./...