Fixed container name regex to match Docker Remote API spec

The previous container name validation was for a valid image name, not a container name. This patch matches the pattern to what the Docker daemon expects. See the docs for /containers/create here: https://docs.docker.com/reference/api/docker_remote_api_v1.13/#21-containers

> -   **name** – Assign the specified name to the container. Must
>    match `/?[a-zA-Z0-9_-]+`.