Using {{.Task.Slot}} in Docker volumes

Question

I would like to mount individual volumes to each replica of my Docker service using the {{.Task.Slot}} syntax:

services:
  foo:
    ...
    volumes:
      - type: volume
        source: foo{{.Task.Slot}}
        target: /mnt
    deploy:
      mode: replicated
      replicas: 3

volumes:
  foo1:
    ...
  foo2:
    ...
  foo3:
    ...

However, Docker fails with:

service foo: undefined volume "foo{{.Task.Slot}}"

It seems that the Go syntax is not interpreted in the source property but in the target property, it works smoothly:

services:
  foo:
    ...
    volumes:
      - type: volume
        source: foo1
        target: /mnt{{.Task.Slot}}

But that's obviously not what I need.

Answer 1

This is the correct way to do it:

services:
  foo:
    ...
    volumes:
      - foo:/mnt
    deploy:
      mode: replicated
      replicas: 3

volumes:
  foo:
    name: 'foo-{{.Task.Slot}}'
    ...

Scaling the service will then create the volume(s) as needed.

All credits go to @larsks.