Allow to download artifacts from the container

[EugenKon]

Proposal

There is a problem to fetch the template from the container we want to run: #23416 (comment)

Use-cases

It would be nice to keep the template inside the container which we want to run. Thus, depending on the container we want to run it could be possible to store it own template.

      driver = "docker"

      config {
        force_pull = false
        image = "xxx/nginx"
        volumes = [
          "custom/internal.nginx.conf:/tmp/nginx/sites-enabled/site",
        ]
      }

        artifact {
          source      = "docker://some/path/inside/container/nginx.tpl"
          destination = "local/nginx.tpl"
        }

        template {
          source        = "local/nginx.tpl"
          destination   = "custom/internal.nginx.conf"
          change_mode   = "signal"
          change_signal = "SIGHUP"
       }

Attempted Solutions

I was tried to create pre-start task which fetches the template and make it available via /alloc. But /alloc is not mounted into /opt/nomad/data/<ID>/<task>/alloc folder for tasks with docker driver.

[tgross]

@EugenKon the artifact and template blocks run outside of the task. They currently run in the client's context, but our long-term vision is to have them run as tasks themselves. We can't run things like template or artifact inside your tasks unless we mount consul-template binaries (or something like that) inside the task container. This wouldn't work for anything except Docker containers.

A different way to solve the problem that you've got in #23416 (comment) is to run consul-template as PID1 inside your Nginx container. I'd recommend trying something like that.

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