Add a osx release target to the makefile to ease releasing.

Signed-off-by: Jean-Laurent de Morlhon <jeanlaurent@morlhon.net>

Makefile

@@ -1,6 +1,7 @@
 .PHONY: all deps osxkeychain secretservice test validate wincred
 
 TRAVIS_OS_NAME ?= linux
+VERSION = 0.4.2-dev
 
 all: test
 
@@ -16,6 +17,10 @@ codesign: osxkeychain
 	xcrun -log codesign -s $(SIGNINGHASH) --force --verbose bin/docker-credential-osxkeychain
 	xcrun codesign --verify --deep --strict --verbose=2 --display bin/docker-credential-osxkeychain
 
+osxrelease: codesign
+	mkdir -p release
+	cd bin && tar cvfz ../release/docker-credential-osxkeychain-v$(VERSION)-amd64.tar.gz docker-credential-osxkeychain
+
 secretservice:
 	mkdir -p bin
 	go build -o bin/docker-credential-secretservice secretservice/cmd/main_linux.go