Docker API: empty or null Cmd prevents inheritance of the image’s default command #108

Description

Containers exit immediately in VMPal when a Docker API request includes an empty command ("Cmd": []) or "Cmd": null, even when the image defines a default command.

Redis is used below as a minimal example.

Steps to reproduce

With VMPal Containers enabled:

docker --context vmpal pull redis:8-alpine

curl --silent --show-error \
  --unix-socket "$HOME/.vmpal/docker.sock" \
  -H 'Content-Type: application/json' \
  -X POST \
  'http://localhost/containers/create?name=empty-command-test' \
  --data-binary \
  '{"Image":"redis:8-alpine","Cmd":[],"HostConfig":{"AutoRemove":false}}'

docker --context vmpal start empty-command-test

docker --context vmpal inspect empty-command-test \
  --format 'Command={{json .Config.Cmd}} Status={{.State.Status}} ExitCode={{.State.ExitCode}}'

Actual behavior

Command=[] Status=exited ExitCode=0

Redis never starts. The same happens with "Cmd": null.

Omitting Cmd, or explicitly setting "Cmd": ["redis-server"], starts Redis successfully.

Expected behavior

When no entrypoint override is supplied, the container should inherit the image’s default command.

For this example, the effective command should be ["redis-server"], and Redis should start.

This matches Docker Engine’s implementation, which falls back to the image command when the request’s command is empty.

This reproduction uses the Docker API directly. It does not involve Compose, whose command: [] setting has different semantics.

Versions tested

• VMPal 0.112 (112)
• VMPal container engine: 29.0.0-vmpal, API 1.52
• Example image: redis:8-alpine