package.versions answered installedVersion "15.17" for btcpay-server while
offering "2.4.2" — 15.17 being its postgres dependency's tag. With BTCPay's
own container absent, installed_version fell back to `containers.first()`,
which for a multi-container stack is an arbitrary sibling.
That is the number the update decision is made from, and it is what the UI
shows next to the available version, so a nonsense pair like "installed 15.17,
available 2.4.2" is presented as a legitimate upgrade.
The fallback now only applies when there is exactly one container, which still
covers apps whose container is named differently from their id (immich_server
for immich). With several containers and no identifiable backend, the honest
answer is "unknown" rather than a guess at a sibling.
Extracted as select_backend_container so the rule is testable directly.
Tests: the BTCPay stack case, the lone differently-named container, and the
archy- prefixed preference. Full suite 1157/1157.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
