[BUG] Negative replica count in scale flags (docker compose scale / --scale) is accepted without validation #14482

Description

According to the Compose Specification, scale must be a non-negative integer (>= 0). The Compose file loader already enforces this and rejects negative scale: values in compose.yaml.

Additionally, the CLI properly validates non-numeric inputs — for instance, docker compose scale web=abc immediately returns:

invalid scale specifier: can't parse replica value as int: web=abc

However, when passing a negative replica count through the CLI flags, such as docker compose scale web=-1 or docker compose up --scale web=-1, the value is accepted without any validation error. The service is then assigned a negative replica count, which leads to unexpected behavior during convergence and container lifecycle handling.

The CLI should validate that the replica count is non-negative, consistent with the Compose Specification and with how the compose file loader treats negative scales.

Steps To Reproduce

1. Create a minimal compose.yaml:

   services:
     web:
       image: alpine

2. Attempt to scale the service with a negative replica count:

   docker compose scale web=-1

   Or:

   docker compose up --dry-run --scale web=-1

Observed Result

• docker compose scale web=-1:

  [+] scale 1/1
   ✔ Network a_default Created                                                                                    0.0s
  service "web" has no container to start
  

  The negative value -1 is accepted without any validation error.

• docker compose up --dry-run --scale web=-1:

  end of 'compose up' output, interactive run is not supported in dry-run mode
  

  The command finishes without warning or error regarding the negative replica count.

Compose Version

$ docker compose version
Docker Compose version v5.5.0

Docker Environment

Docker version 29.9.3, build 5457199aa5
Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.17.2
    Path:     /path/to/.docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v5.5.0
    Path:     /path/to/.docker/cli-plugins/docker-compose

Anything else?

No response