Pass -sign parameter to build.go from Docker if key is present

build.sh

@@ -74,27 +74,33 @@ case "${1:-default}" in
 		;;
 
 	all)
-		build -goos darwin -goarch amd64 tar
+		if [ -f /etc/syncthing/syncthing.priv ] ; then
+			# Default signing key location. If present, use it to sign the
+			# release.
+			extra=(-sign /etc/syncthing/syncthing.priv)
+		fi
+
+		build -goos darwin -goarch amd64 ${extra[@]-} tar
 
-		build -goos dragonfly -goarch amd64 tar
+		build -goos dragonfly -goarch amd64 ${extra[@]-} tar
 
-		build -goos freebsd -goarch 386 tar
-		build -goos freebsd -goarch amd64 tar
+		build -goos freebsd -goarch 386 ${extra[@]-} tar
+		build -goos freebsd -goarch amd64 ${extra[@]-} tar
 
-		build -goos linux -goarch 386 tar
-		build -goos linux -goarch amd64 tar
-		build -goos linux -goarch arm tar
+		build -goos linux -goarch 386 ${extra[@]-} tar
+		build -goos linux -goarch amd64 ${extra[@]-} tar
+		build -goos linux -goarch arm ${extra[@]-} tar
 
-		build -goos netbsd -goarch 386 tar
-		build -goos netbsd -goarch amd64 tar
+		build -goos netbsd -goarch 386 ${extra[@]-} tar
+		build -goos netbsd -goarch amd64 ${extra[@]-} tar
 
-		build -goos openbsd -goarch 386 tar
-		build -goos openbsd -goarch amd64 tar
+		build -goos openbsd -goarch 386 ${extra[@]-} tar
+		build -goos openbsd -goarch amd64 ${extra[@]-} tar
 
-		build -goos solaris -goarch amd64 tar
+		build -goos solaris -goarch amd64 ${extra[@]-} tar
 
-		build -goos windows -goarch 386 zip
-		build -goos windows -goarch amd64 zip
+		build -goos windows -goarch 386 ${extra[@]-} zip
+		build -goos windows -goarch amd64 ${extra[@]-} zip
 		;;
 
 	test-cov)