providers/docker: be more lenient about deleting built image

plugins/providers/docker/action/destroy_build_image.rb

@@ -26,7 +26,10 @@ module VagrantPlugins
 
           if image
             machine.ui.output(I18n.t("docker_provider.build_image_destroy"))
-            machine.provider.driver.rmi(image)
+            if !machine.provider.driver.rmi(image)
+              machine.ui.detail(I18n.t(
+                "docker_provider.build_image_destroy_in_use"))
+            end
           end
 
           if image_file && image_file.file?

plugins/providers/docker/driver.rb

@@ -104,7 +104,9 @@ module VagrantPlugins
 
       def rmi(id)
         execute('docker', 'rmi', id)
+        return true
       rescue Exception => e
+        return false if e.to_s.include?("is using it")
         raise if !e.to_s.include?("No such image")
       end
 

templates/locales/providers_docker.yml

@@ -4,6 +4,10 @@ en:
       Image is already built from the Dockerfile. `vagrant reload` to rebuild.
     build_image_destroy: |-
       Removing built image...
+    build_image_destroy_in_use: |-
+      Build image couldn't be destroyed because the image is in use. The
+      image must be destroyed manually in the future if you want to remove
+      it.
     build_image_invalid: |-
       Build image no longer exists. Rebuilding...
     building: |-