The SPA was rendering <base href="https://semaphore.local.mk-labs.cloud/">
regardless of which hostname served the page, causing the imagineering
alias to load the same UI but rewrite all in-page navigation back to the
semaphore hostname. Confusing for users hitting either Traefik alias.
Leave SEMAPHORE_WEB_ROOT empty so Semaphore emits relative URLs and is
fully host-agnostic. Both DNS names (semaphore + imagineering) now serve
cleanly without preference between them.
The template only emits SEMAPHORE_WEB_ROOT when the variable is set, so
the env-var is absent from the container when not needed.
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