Verifies that a configured resolver drives the proxy's outbound DNS
resolution. A gost HTTP proxy uses a resolver whose only nameserver is a
test responder answering echo.test with the real echo server IP and
NXDOMAIN for everything else. A request to echo.test is resolved by the
custom resolver and reaches the echo server; an unmapped host fails to
resolve. Adds a parametrized DNS responder script and container helper.
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