Tests that require `not-rootless` are getting skipped with `target=docker` (where the cli is not run as root)

### Description

`rootlessutil.IsRootless` will (appropriately) return true if we are not root - but then, the docker daemon may run rootful, and at least some of these tests should definitely run for docker as well.

While the CI is running with `-exec sudo` - running locally without `sudo` will skip tests and give the developer the false impression it is fine.

eg:

```
go test -v ./cmd/nerdctl/container/ -run TestRunWithOOMScoreAdj -test.target docker
go test -v -exec sudo ./cmd/nerdctl/container/ -run TestRunWithOOMScoreAdj -test.target docker
```

Should likely yield the same results.

Pretty sure there are side-effects to consider here, as not being root will prevent us from writing certain files on the host for example, so, we should not change that on the legacy tooling - but we should account for that in the tooling rewrite.

### Steps to reproduce the issue

na

### Describe the results you received and expected

na

### What version of nerdctl are you using?

main

### Are you using a variant of nerdctl? (e.g., Rancher Desktop)

None

### Host information

_No response_