Carto Voyager was returning 'API Key required' content for the
user (likely rate-limited or IP-blocked by Carto). Switched to
ESRI World_Street_Map which has CORS-friendly tiles with plenty
of place names and street labels.
URL format is different from Carto: {z}/{y}/{x}.jpg (y before x,
jpg not png). Updated the build to match. The local /esri/
proxy in default.conf handles the upstream CORS-incompatibility
by adding permissive CORS headers and a proper User-Agent.
Richard Zink
4500fe3214