Add spec file support for Windows plugin discovery.

Plugin discovery on Windows is not possible using named pipes. However,
it is possible using spec file (tcp based). This adds Windows specific
paths for discovery.

Fixes #23605

Signed-off-by: Anusha Ragunathan <anusha@docker.com>

pkg/plugins/discovery.go

@@ -16,7 +16,6 @@ var (
 	// ErrNotFound plugin not found
 	ErrNotFound = errors.New("plugin not found")
 	socketsPath = "/run/docker/plugins"
-	specsPaths  = []string{"/etc/docker/plugins", "/usr/lib/docker/plugins"}
 )
 
 // localRegistry defines a registry that is local (using unix socket).

pkg/plugins/discovery_unix.go

@@ -0,0 +1,3 @@
+package plugins
+
+var specsPaths = []string{"/etc/docker/plugins", "/usr/lib/docker/plugins"}

pkg/plugins/discovery_windows.go

@@ -0,0 +1,8 @@
+package plugins
+
+import (
+	"os"
+	"path/filepath"
+)
+
+var specPaths = []string{filepath.Join(os.Getenv("programdata"), "docker", "plugins")}