New 'image' lookup is the SPOT for resolving <registry>/<image>:<version>
(swarm registry prefix in swarm mode, bare in compose; custom= accepts a
string base for a second distinct custom image inside one app).
container_image becomes a thin wrapper that forwards terms+kwargs and adds
the compose image: "<ref>" line, so all existing template call sites keep
their output byte-identical. The lookup-usage integration test now counts
lookup_loader.get() delegation as production usage.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>