build: Run validate headers in build step

Signed-off-by: Chris Crone <christopher.crone@docker.com>

Dockerfile

@@ -27,6 +27,7 @@ ARG GOLANGCI_LINT_VERSION=v1.33.0-alpine
 FROM --platform=${BUILDPLATFORM} golang:${GO_VERSION} AS builder
 WORKDIR /go/src/github.com/docker/hub-tool
 RUN apk add --no-cache \
+    bash \
     git \
     make
 
@@ -51,6 +52,14 @@ RUN --mount=type=cache,target=/root/.cache/go-build \
     --mount=type=cache,target=/root/.cache/golangci-lint \
     make -f builder.Makefile lint
 
+####
+# VALIDATE HEADERS
+####
+FROM builder AS validate-headers
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg \
+    go get -u github.com/kunalkushwaha/ltag && ./scripts/validate/fileheader
+
 ####
 # CHECK GO MOD
 ####

Makefile

@@ -111,9 +111,7 @@ lint: ## Run the go linter
 
 .PHONY: validate-headers
 validate-headers: ## Validate files license header
-	docker run --rm -v $(CURDIR):/work -w /work \
-		golang:${GO_VERSION} \
-		bash -c 'go get -u github.com/kunalkushwaha/ltag && ./scripts/validate/fileheader'
+	@docker build $(BUILD_ARGS) . --target validate-headers
 
 .PHONY: validate-go-mod
 validate-go-mod: ## Validate go.mod and go.sum are up-to-date