Updating the image of a service named like "api.test" wrote a nested
"test" key into the "api" service, because data_get/data_set split the
name on dots. Use array key paths instead. Also refuse service names that
Docker cannot use as a container name prefix, with a clear error on save.
Closes#12456
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
