gvisor build in rootless docker

### Description

Don't think this ever worked hence reporting as a feature request. It's a bit of both really.

Anyway: I run docker rootless and it's major pain getting gvisor to build.

### Is this feature related to a specific bug?

_No response_

### Do you have a specific solution in mind?

I managed to get it working by commenting out https://github.com/google/gvisor/blob/1ea26f133476461497b359f602fc3cd8977b5a05/tools/bazel.mk#L308 and running `make` with a bunch of extra flags:

```
make copy TARGETS=runsc DESTINATION=/tmp \
USER=root \
DOCKER_PRIVILEGED= \
BAZEL_CACHE_USE_VOLUME=true \
BAZEL_CACHE=/root/.cache/bazel
```

and then I still had to `docker exec -it gvisor-bazel-6d5dbb77-x86_64 /bin/bash` and manually fish the `runsc` binary out of `bazel-bin/…` because it didn't get copied to /tmp despite it being mounted.

Would be nice to make this work out of the box, or at least document this.