Follow #242.
Move `docker.host` to `container.docker_host`.
There are already some options for docker/container in `container`, so developers could get confused about where to add options.
It's breaking, but I think it's OK since `docker.host` was added just two days ago.
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/244
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>