With a variable upstream ($ui_upstream, added for docker-DNS re-resolution),
a trailing-slash URI in proxy_pass makes nginx forward '/' to the UI for every
sub-path, so /_next/static/*.css and /i/<invoice> all return the app index HTML
(200 text/html) -> the UI renders unstyled and client routing breaks. Omit the
URI to preserve the full request path, matching the /api/ block.
Mirror of wowcheckout e0c349a. Same bug would bite xmrcheckout after any api container rebuild — nginx caches IP at startup and never re-resolves, so a rebuild that gives the api container a new IP leaves nginx 502'ing until manually restarted. Use 'resolver 127.0.0.11 valid=10s' + variable-based proxy_pass so nginx re-resolves every ~10s via Docker's internal DNS.
jwinterm