mara asked whether SWARM_CONTROLLER_PUBLIC_URL should also be set when the
controller does not run on the same host, and then said the two read as one
service to her. Both are worth answering in the file rather than only in the
thread: that split is not representable today, and the reason is two hops
away from this line.
The UI's /api/ location proxies http://unix:<socketPath> -- a path that
resolves nowhere else -- and this daemon binds no TCP address at all. So the
vhost and the daemon are co-located by construction, and ui.enable is the
flag that declares the vhost rather than a guess about some host serving it.
Also names the tripwire: the day the daemon grows a TCP listener, this line
silently stops being right (env unset, registration quietly skipped, no
error). That is when an explicit publicUrl option becomes correct -- not
before, while there is exactly one derivable answer.
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