61f6c927 removed the Dialer for unix
socket. This was done because a change to the GRPC library caused the
Dialer to stop working, and because the default implementation works as
expected for users not using a proxy.

Unfortunately this led to a regression for users with HTTP or
HTTPS proxy configurations exposed via the `http_proxy` or `https_proxy`
environment variables.

For this reason, we reintroduce the dialer for Unix socket connections.