build: add release-snapshot Make target

This allows you to run the release locally, without publishing or
signing, against the current state of the repository.

There are some more improvements I would like to make to the
`Makefile` e.g., the deprecation of `golint` and the introduction of a
`help` target. But they are out of scope for the current things I am
working on.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

Makefile

@@ -2,11 +2,20 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-PROJECT := github.com/getsops/sops/v3
-GO      := GOPROXY=https://proxy.golang.org go
-GOLINT  := golint
+PROJECT            := github.com/getsops/sops/v3
+PROJECT_DIR        := $(shell dirname $(abspath $(lastword $(MAKEFILE_LIST))))
+GO                 := GOPROXY=https://proxy.golang.org go
+GOLINT             := golint
 
+GITHUB_REPOSITORY  ?= github.com/getsops/sops
+
+GORELEASER         := $(PROJECT_DIR)/bin/goreleaser
+GORELEASER_VERSION ?= v1.20.0
+
+.PHONY: all
 all: test vet generate install functional-tests
+
+.PHONY: origin-build
 origin-build: test vet generate install functional-tests-all
 
 install:
@@ -18,34 +27,54 @@ tag: all
 lint:
 	$(GOLINT) $(PROJECT)
 
+.PHONY: vendor
 vendor:
 	$(GO) mod tidy
 	$(GO) mod vendor
 
 vet:
 	$(GO) vet $(PROJECT)
 
+.PHONY: test
 test: vendor
 	gpg --import pgp/sops_functional_tests_key.asc 2>&1 1>/dev/null || exit 0
 	./test.sh
 
 showcoverage: test
 	$(GO) tool cover -html=coverage.out
 
+.PHONY: generate
 generate: keyservice/keyservice.pb.go
 	$(GO) generate
 
 %.pb.go: %.proto
 	protoc --go_out=plugins=grpc:. $<
 
+.PHONY: functional-tests
 functional-tests:
 	$(GO) build -o functional-tests/sops github.com/getsops/sops/v3/cmd/sops
 	cd functional-tests && cargo test
 
-# Ignored tests are ones that require external services (e.g. AWS KMS)
-# 	TODO: Once `--include-ignored` lands in rust stable, switch to that.
+.PHONY: functional-tests-all
 functional-tests-all:
 	$(GO) build -o functional-tests/sops github.com/getsops/sops/v3/cmd/sops
+	# Ignored tests are ones that require external services (e.g. AWS KMS)
+	# 	TODO: Once `--include-ignored` lands in rust stable, switch to that.
 	cd functional-tests && cargo test && cargo test -- --ignored
 
-.PHONY: all test generate clean vendor functional-tests
+.PHONY: release-snapshot
+release-snapshot: install-goreleaser
+	GITHUB_REPOSITORY=$(GITHUB_REPOSITORY) $(GORELEASER) release --clean --snapshot --skip-sign
+
+.PHONY: install-goreleaser
+install-goreleaser:
+	$(call go-install-tool,$(GORELEASER),github.com/goreleaser/goreleaser@$(GORELEASER_VERSION),$(GORELEASER_VERSION))
+
+# go-install-tool will 'go install' any package $2 and install it to $1.
+define go-install-tool
+@[ -f $(1)-$(3) ] || { \
+set -e ;\
+GOBIN=$$(dirname $(1)) go install $(2) ;\
+touch $(1)-$(3) ;\
+}
+endef