Hi,
LXD takes images from https://images.linuxcontainers.org , but gives no hint where to report bugs of the images to. (Neither LXD software bug, nor upstream bug, mistake in image building. Here: alpine/3.6)
Would that be a bug report at https://github.com/lxc/lxc-ci ?
regards
Answer is “it depends” 
Usually the right place is https://github.com/lxc/lxc/issues as that’s the repository which contains the shell scripts (under templates/) which are used to generate the root filesystem.
If it’s some metadata or cleanup that went bad, then https://github.com/lxc/lxc-ci/issues is more appropriate.