Added some clarifications to how push will behave

Author: toote

README.md

@@ -32,7 +32,9 @@ The name of the service the command should be run within. If the docker-compose
 
 A list of services to push.  You can specify just the service name to push or the format `service:registry:tag` to override where the service's image is pushed to. Needless to say, the image for the service must have been built in the very same step or built and pushed previously to ensure it is available for pushing.
 
-:warning: The `push` command will fail if a `login` command has not previously been used or the agent has not been authenticated for an image registry.
+:warning: If a service does not have an `image` configuration and no registry/tag are specified in the `push` option, pushing of the service will be skipped by docker.
+
+:warning: The `push` command will fail when the image refers to a remote registry that requires a login and the agent has not been authenticated for it (for example, using the [ecr](https://github.com/buildkite-plugins/ecr-buildkite-plugin) or [docker-login](https://github.com/buildkite-plugins/docker-login-buildkite-plugin) plugins).
 
 ### Other options