Split build and push, and remove push code from run

This currently only changes the `gcp` target, but is the new
model - the `build` command will only do things locally, then
you need to `push` to an image store such as GCP or other ones
in order to `run` for platforms that cannot boot directly from
a local image.

Fix #1618

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

examples/gcp.yml

@@ -65,8 +65,4 @@ trust:
     - linuxkit/kernel
 outputs:
   - format: kernel+initrd
-  - format: gcp
-    project: moby
-    bucket: mobytestjustin
-    family: moby-dev
-    replace: true
+  - format: gcp-img