Add docker prefix to image (#58)

* Add docker prefix

* Add changeset

* Add detail to changeset

Author: apmatthews

.changeset/chilly-timers-drive.md

@@ -0,0 +1,5 @@
+---
+"@wpengine/github-action-wpe-site-deploy": patch
+---
+
+Fixes an issue in v3.2.1 where the action may fail to resolve the public Docker image

action.yml

@@ -46,7 +46,7 @@ inputs:
 
 runs:
   using: "docker"
-  image: wpengine/site-deploy:1.0.0
+  image: docker://wpengine/site-deploy:1.0.0
   env:
     WPE_SSHG_KEY_PRIVATE: ${{ inputs.WPE_SSHG_KEY_PRIVATE }}
     WPE_ENV: ${{ inputs.WPE_ENV }}