docker: a plain docker build . stamps the git version (closes #210)

A plain `docker build .`, which is how upaas builds, stamped `dev`:
`.dockerignore` left out `.git` and the builder declared
`ARG VERSION=dev`. `.dockerignore` now sends `.git` and lists no tracked
file, which git in the build would count as deleted and mark `-dirty`.
`ARG VERSION` has no default. The Makefile takes `VERSION` from the
environment with `?=`, so a build arg still wins (`script/docker` keeps
passing one); otherwise `git describe` runs in the builder. A new
`make version` prints the version, and the builder fails when the
context carries `.git` and it comes out empty, `dev` or `unknown`.

Model: opus-5-5

Makefile

@@ -1,9 +1,10 @@
-.PHONY: all bootstrap setup build lint fmt fmt-check test check clean hooks docker
+.PHONY: all bootstrap setup build version lint fmt fmt-check test check clean hooks docker
 
 BINARY := dnswatcher
-# `make build VERSION=...` overrides this; the Dockerfile does so, as the
-# image has no .git to describe.
-VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
+# VERSION given on the command line (`make build VERSION=...`) or in the
+# environment, which is how the Dockerfile's VERSION build arg arrives,
+# wins over what `git describe` says of this checkout.
+VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
 LDFLAGS := -X main.Version=$(VERSION)
 
 # Standard targets are thin shims; the implementations live in script/
@@ -21,6 +22,10 @@ setup:
 build:
 	go build -ldflags "$(LDFLAGS)" -o bin/$(BINARY) ./cmd/dnswatcher
 
+# Prints the version `make build` stamps; the Dockerfile checks it.
+version:
+	@echo "$(VERSION)"
+
 test:
 	@script/test