Root cause of the 'waiting for connection' build failure: the act_runner
executor container runs on the same docker network as buildkitd, so its
127.0.0.1 is its own loopback — the host-published port is unreachable
from there. Revert to tcp://<name>:<port>; container DNS resolves on the
shared network. Drop the now-useless loopback port publish.
Verified on host: from a container on that network, endpoint
tcp://<name>:8375 -> 'Status: running'; tcp://127.0.0.1:8375 ->
'ERROR: context deadline exceeded' (exact CI symptom).