Update documentation regarding Bazel and Docker TCP connections

[Knut Eirik Leira Hjelle]

The documentation mentions --test_env=DOCKER_HOST='unix://<path_to_sock>', but then I tried specifying a TCP connection I got an error message that at least got me stuck;

error loading oci layout into daemon: error loading image: Error response from daemon: Client sent an HTTP request to an HTTPS server.

Specyfing the following made it work
--test_env=DOCKER_TLS_VERIFY=1
--test_env=DOCKER_CERT_PATH=/path/to/certs