container detection fails inside `docker buildx`

[cgwalters]

I was playing around with GH actions for container building, it uses docker buildx by default and the environment there appears to lack /.dockerenv.

Hmmm. It's a bit tempting to force on setting the container environment variable in ostree native containers.

[vrutkovs]

I've hit this in github actions in https://github.com/vrutkovs/custom-okd-os and the fix was to add
ENV container oci in the dockerfile