bug#35359: docker-image produces corrupt tar

[rendaw]

Package: guix
Version: 0.16.0

I took the sample system configuration at the top of
https://www.gnu.org/software/guix/manual/en/html_node/Using-the-Configuration-System.html#Using-the-Configuration-System
and ran:

```
$ guix system docker-image testsystem2.scm --on-error=backtrace
successfully built
/gnu/store/l3psqh73xjb5h11y1hwrlfvl1yr920x4-guixsd-docker-image.tar.gz.drv
/gnu/store/giaw69wla9zdf4k7nddyyglvhq3629jq-guixsd-docker-image.tar.gz
$ tar -ztf
/gnu/store/d8rr8ilwm7l0xjachp7glbhp9qf4hjyg-guixsd-docker-image.tar.gz ^C
$ docker load <
/gnu/store/giaw69wla9zdf4k7nddyyglvhq3629jq-guixsd-docker-image.tar.gz
Error processing tar file(exit status 1): unexpected EOF
$ tar -ztf
/gnu/store/giaw69wla9zdf4k7nddyyglvhq3629jq-guixsd-docker-image.tar.gz
./
./41bc17c3316805da9a209f5835327160c8993ab4311c9cb649911a4ae41d1603/
./41bc17c3316805da9a209f5835327160c8993ab4311c9cb649911a4ae41d1603/VERSION
./41bc17c3316805da9a209f5835327160c8993ab4311c9cb649911a4ae41d1603/json
./41bc17c3316805da9a209f5835327160c8993ab4311c9cb649911a4ae41d1603/layer.tar

gzip: stdin: unexpected end of file
tar: Unexpected EOF in archive
tar: Error is not recoverable: exiting now
```

Qemu (?) clears my scroll history but I don't see any errors or warnings
from that point on.

My system is pretty typical - x86 Arch Linux with standard core binaries
and such.