Change compose file handling to require valid service specifications

Added the checks and some tests
One of the tests wasn't valid docker-compose.yml, that's been changed.
Bad config throws an error and has a test

Signed-off-by: Jack Laxson <jackjrabbit@gmail.com>

hack/demo-env/examples/compose/docker-compose.yml

@@ -7,6 +7,7 @@ services:
     image: docker.io/tonistiigi/db
   webapp:
     build:
+      context: .
       dockerfile: Dockerfile.webapp
       args:
         buildno: 1