Update openssl command in the Makefile

Makefile

@@ -65,7 +65,7 @@ release: binclean gox-build
 .PHONY: gox-build
 gox-build: $(GOPATH)/bin/gox
 	CGO_ENABLED=0 $(GOPATH)/bin/gox -output="_release/docker-builder-$(REPO_VERSION)-{{ .OS }}-{{ .Arch }}" -osarch="darwin/amd64 linux/amd64" $(GOBUILD_VERSION_ARGS) $(GO_TAG_ARGS) $(B)
-	for file in $$(find ./_release -type f -name 'docker-builder-*') ; do openssl sha256 -out $$file-SHA256SUM $$file ; done
+	for file in $$(find ./_release -type f -name 'docker-builder-*') ; do openssl dgst -sha256 -out $$file-SHA256SUM $$file ; done
 
 .PHONY: .test
 .test: fmtpolice bats