Update/fix build tags, Dockerfile, and release.sh for proper building and releasing of linux/386 and linux/arm cross-compiled client binaries

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

pkg/mount/flags_linux.go

@@ -1,3 +1,5 @@
+// +build amd64
+
 package mount
 
 import (

pkg/mount/flags_unsupported.go

@@ -1,4 +1,5 @@
-// +build: !linux !amd64
+// +build !linux !amd64
+
 package mount
 
 func parseOptions(options string) (int, string) {

pkg/mount/mounter_linux.go

@@ -1,3 +1,5 @@
+// +build amd64
+
 package mount
 
 import (

pkg/mount/mounter_unsupported.go

@@ -1,4 +1,5 @@
-// +build: !linux !amd64
+// +build !linux !amd64
+
 package mount
 
 func mount(device, target, mType string, flag uintptr, data string) error {