Support Rootless Docker

[Alexhuszagh]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

Currently, if using rootless docker, cross fails to run. This is because we automatically add --user 1000:1000 permissions (or the current UID/GID) when running Docker. This is an issue, because there is now rootless docker:

First, install rootless docker:

$  dockerd-rootless-setuptool.sh install  # this may require a --force if the rootful docker is available.

Then, use the rootless context and try to touch a file:

$ docker context use rootless 
rootless
Current context is now "rootless"
$ docker run --user 1000:1000 -it --rm -v "$PWD":/project -w /project ubuntu:20.04 bash
groups: cannot find name for group ID 1000
$ touch a
touch: cannot touch 'a': Permission denied

This can be solved by allowing an environment variable to override our default, good assumptions of whether the container engine is rootful or not.

Describe why this would be a good inclusion for cross

Currently, detecting rootful/rootless mode is quite difficult, or expensive computationally, and the defaults are quite good:

• Podman always runs rootless
• Docker mostly runs rootful

Therefore, just making these assumptions generally works. However, this is an issue if rootless docker exists, or we have another container engine that runs as root. In short, we need to be able to override setting --user 1000:1000 permissions. This likely could best be done with a CROSS_ROOTLESS_CONTAINER_ENGINE, which is an Option<bool>, parsed via bool_from_envvar. If it's not present, use the sensible default. If it is present, force the presence or absence of lower user permissions.

This also should simplify supporting new container engines, since we can handle those with varying behavior quite easily, without any code changes, until we can provide reasonable defaults for them, such as in #588.