The front resolves its upstreams from *_ADDRESS, defaulting to the bare
compose service names — admin, antispam — and reads the 1.9-era HOST_*
not at all. Phase 1 never noticed because the predecessor's service
names WERE the defaults; the mesh's containers are mailu-*, and the
front answered 502 asking docker for a name nothing carries. The dead
vocabulary goes; every upstream is named as the container actually is.
jschoubben