docker_api_attach_test: fix WS test to use DOCKER_TEST_HOST if specified

TestGetContainersAttachWebsocket is currently broken on Windows CI tests
b/c it has hardcoded unix://var/run/docker.sock.  This change makes use
of @icecrime's code in docker_utils and generalizes it with sockConn()
to provide a net.Conn by making use of DOCKER_TEST_HOST. Also fixes
the test.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>