Add tmpfs option to gitlab-runner configuration

This is a feature request to support tmpfs for the docker runner without allowing privileged usage of the containers.

Idea: Mount /tmp to RAM to reduce disk access

Use case: We are building pdf files with a LaTeX container, which generates many files in /tmp. These are necessary for the build-process. But we want to reduce disk access by mounting a directory, like /tmp, to the RAM via tmpfs.

This can already be used with the docker run command as seen here:

docker run -d --tmpfs /run:rw,noexec,nosuid,size=65536k my_image

and could also be achieved when allowing privileged usage of containers, but this disables security mechanisms of the containers.

It would be awesome to have this feature in the configuration like this:

[[runners]]
  name = "default"
  url = "ci"
  token = "token"
  executor = "docker"
  [runners.docker]
    privileged = false
    tmpfs = ["/tmp:rw,noexec,nosuid,size=65536k"]
    [...]

Thanks!