The `--rootless` flag had a couple of issues:
* #38702: euid=0, $USER="root" but no access to cgroup ("rootful" Docker in rootless Docker)
* #39009: euid=0 but $USER="docker" (rootful boot2docker)
To fix#38702, XDG dirs are ignored as in rootful Docker, unless the
dockerd is directly running under RootlessKit namespaces.
RootlessKit detection is implemented by checking whether `$ROOTLESSKIT_STATE_DIR` is set.
To fix#39009, the non-robust `$USER` check is now completely removed.
The entire logic can be illustrated as follows:
```
withRootlessKit := getenv("ROOTLESSKIT_STATE_DIR")
rootlessMode := withRootlessKit || cliFlag("--rootless")
honorXDG := withRootlessKit
useRootlessKitDockerProxy := withRootlessKit
removeCgroupSpec := rootlessMode
adjustOOMScoreAdj := rootlessMode
```
Close#39024Fix#38702#39009
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
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