[docker] Expand variables in volume destinations

What does this MR do?

Expand variables in volume destinations

Why was this MR needed?

Variables in volume destinations are expanded, so that the functionality aligns with the one from the kubernetes executor.

What's the best way to test this MR?

• config.toml

  listen_address = ":9252"
  concurrent = 4
  check_interval = 0
  connection_max_age = "15m0s"
  shutdown_timeout = 0
  [session_server]
    session_timeout = 1800
  [[runners]]
    name = "docker-local"
    url = "https://gitlab.com/"
    id = 0
    token = "glrt-NopeNopeNope"
    token_obtained_at = 0001-01-01T00:00:00Z
    token_expires_at = 0001-01-01T00:00:00Z
    executor = "docker"
    shell = "bash"
    environment = [ "COUNTER_DIR=/tmp/foo/bar" ]
    [runners.docker]
      tls_verify = false
      image = ""
      volumes = ["/${COUNTER_DIR}"]

• .gitlab-ci.yaml

  test:
    image: alpine
    script:
    - |
      set -x
      counter=0
      counterFile="${COUNTER_DIR}/counter"
      
      if test -e "${counterFile}" ; then
        counter="$(cat "${counterFile}")"
      else
        echo >&2 "## no counter yet; fresh volume mount"
      fi
  
      counter=$(( counter + 1 ))
      echo "${counter}" > "${counterFile}"
      echo >&2 "## wrote new counter ${counter} to ${counterFile}"

• run the pipeline once and see that:

  • a new volume is created: docker volume ls
  • the counter file cannot be found (new, fresh volume)
  • the counter file can be written to

• run the job again and see that:

  • the counter file can be read: the same volume is reused, and because we the counter file is written and read from ${COUNTER_DIR} also shows that the volume mount point is expanded.

What are the relevant issue numbers?

closes: #38249 (closed)