$ lxc launch ubuntu-daily:bionic b1792905
Creating b1792905
Starting b1792905
$ lxc exec b1792905 -- cloud-init status --wait

status: done
$ lxc exec b1792905 -- dpkg-query --show cloud-initramfs-copymods
cloud-initramfs-copymods	0.40ubuntu1

## nothing in the image currently owns /lib/modules and it does not exist.
$ lxc exec b1792905 -- dpkg -S /lib/modules
dpkg-query: no path found matching pattern /lib/modules

$ lxc exec b1792905 -- ls -l /lib/modules
ls: cannot access '/lib/modules': No such file or directory


## enable proposed
$ echo "deb http://archive.ubuntu.com/ubuntu bionic-proposed main" | lxc file push - b1792905/etc/apt/sources.list.d/proposed.list

$ lxc exec b1792905 -- apt-get update -q
Hit:1 http://archive.ubuntu.com/ubuntu bionic InRelease
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB]
...
Get:38 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 Packages [56.9 kB]
Get:39 http://archive.ubuntu.com/ubuntu bionic-proposed/main Translation-en [25.5 kB]
Fetched 26.4 MB in 7s (3765 kB/s)
Reading package lists...

## install new package.
$ lxc exec b1792905 -- apt-get install cloud-initramfs-copymods
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  libfreetype6
Use 'apt autoremove' to remove it.
The following packages will be upgraded:
  cloud-initramfs-copymods
1 upgraded, 0 newly installed, 0 to remove and 42 not upgraded.
Need to get 3804 B of archives.
After this operation, 2048 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 cloud-initramfs-copymods all 0.40ubuntu1.1 [3804 B]
Fetched 3804 B in 0s (41.7 kB/s)                   
(Reading database ... 28496 files and directories currently installed.)
Preparing to unpack .../cloud-initramfs-copymods_0.40ubuntu1.1_all.deb ...
Unpacking cloud-initramfs-copymods (0.40ubuntu1.1) over (0.40ubuntu1) ...
Setting up cloud-initramfs-copymods (0.40ubuntu1.1) ...
Processing triggers for initramfs-tools (0.130ubuntu3.3) ...

## And see what owns /lib/modules now, and verify it exists.
$ lxc exec b1792905 -- dpkg -S /lib/modules
cloud-initramfs-copymods: /lib/modules
$ lxc exec b1792905 -- ls -l /lib/modules
total 0