Update Docker TLS documentation for disabled cache volumes

Overview

As stated in #4501 (comment 200736957) if the user has disable_cache set to true from the [runners.docker] inside of the config.toml the /certs volume that we suggest to Docker 19.03 TLS since cache volumes are disabled.

Proposal

Add a new example to enable TLS when disable_cache is set to true with following config.toml and .gitlab-ci.yml

[[runners]]
  name = "docker"
  url = "cccc"
  token = "xxx"
  executor = "docker"
  environment = ["DOCKER_TLS_CERTDIR="]
  [runners.custom_build_dir]
  [runners.docker]
    image = "alpine:3.9"
    disable_cache = true
    volumes = [
        "/tmp/certs:/certs"
    ]
    pull_policy = "never"
    shm_size = 0

 image: docker:19.03

 variables:
   DOCKER_DRIVER: overlay2
   DOCKER_TLS_CERTDIR: "/certs/${CI_JOB_ID}" # Create a cert dir per job

 services:
   - docker:19.03-dind

 before_script:
   - echo "$DOCKER_TLS_CERTDIR"
   - docker info

 build:
   stage: build
   script:
     - echo "tada"
     - docker build -t my-docker-image .

Also adding a note in https://gitlab.com/gitlab-org/gitlab-ce/blob/8505049e1f2510642715d9bfd56fc022cd42caf0/doc/ci/docker/using_docker_build.md#L180-183 to explain that disable_cache has to be false would be ideal