Support TCP for CircleCI

[StephanMeijer]

Allocating a remote Docker Engine
Specified using docker layer caching, but build has not been whitelisted.
Contact CircleCI to be whitelisted

Assigned Docker Engine request id: 1234567
  provisioning: ......
Remote Docker engine created. Using VM 'default-UUID'
Created container accessible with:
  DOCKER_CERT_PATH=/tmp/docker-certs1234567
  DOCKER_HOST=tcp://<IP>:2376
  DOCKER_MACHINE_NAME=1234567
  DOCKER_TLS_VERIFY=1
  NO_PROXY=127.0.0.1,localhost,circleci-internal-outer-build-agent,<IP>:2376

Please, not only support UNIX sockets but do support TCP as well.

[inductor]

+1

[tomoyamachi]

Downloading image layers uses https://github.com/genuinetools/reg. I will check it.