specs-go/config.go: Make Root.Readonly omitempty

It's an optional setting, and this change will fix the wordy:

  $ ocitools generate --template <(echo '{}')
  $ grep readonly config.json
      "readonly": false,

Instead, config.json will not contain a 'readonly' entry at all.

Signed-off-by: W. Trevor King <wking@tremily.us>

specs-go/config.go

@@ -71,7 +71,7 @@ type Root struct {
 	// Path is the absolute path to the container's root filesystem.
 	Path string `json:"path"`
 	// Readonly makes the root filesystem for the container readonly before the process is executed.
-	Readonly bool `json:"readonly"`
+	Readonly bool `json:"readonly,omitempty"`
 }
 
 // Platform specifies OS and arch information for the host system that the container