Stamp the build version into the binary (closes #253)

Makefile

@@ -1,8 +1,26 @@
-.PHONY: bootstrap setup assets test lint fmt fmt-check check build run dev deps docker clean hooks css
+.PHONY: bootstrap setup assets test lint fmt fmt-check check build run dev deps docker clean hooks css version
 
 # Default target
 .DEFAULT_GOAL := check
 
+# Version stamped into the binary. Derived from git by script/version;
+# override it (`make build VERSION=v1.2.3`) where git metadata is
+# unavailable, which is how the Dockerfile passes its build arg in.
+VERSION ?= $(shell script/version)
+
+# An empty override (`make build VERSION=`, or a `--build-arg VERSION=`
+# landing on the Dockerfile's `make build VERSION="$VERSION"`) means unset,
+# exactly as it does in script/version -- stamping "" would leave the binary
+# reporting no version and the footer back on its "dev" fallback. `override`
+# is required: a plain assignment loses to the command-line definition it
+# exists to correct.
+override VERSION := $(or $(strip $(VERSION)),$(shell script/version))
+
+# Extra linker flags for the build target. The static relink in the
+# Dockerfile adds -extldflags here rather than passing its own -ldflags,
+# so composing flags cannot drop the version stamp.
+GO_LDFLAGS ?=
+
 bootstrap:
 	@script/bootstrap
 
@@ -28,7 +46,7 @@ check:
 	@script/check
 
 build:
-	go build -o bin/webhooker ./cmd/webhooker
+	go build -ldflags '$(strip -X main.version=$(VERSION) $(GO_LDFLAGS))' -o bin/webhooker ./cmd/webhooker
 
 run: build
 	./bin/webhooker
@@ -40,6 +58,9 @@ deps:
 	go mod download
 	go mod tidy
 
+version:
+	@echo $(VERSION)
+
 docker:
 	@script/docker