$ lxc launch ubuntu-daily:xenial x1792905
Creating x1792905
Starting x1792905
$ lxc exec x1792905 -- cloud-init status --wait
..
status: done
$ lxc exec x1792905 -- dpkg-query --show cloud-initramfs-copymods
cloud-initramfs-copymods	0.27ubuntu1.5

## nothing in the image currently owns /lib/modules.
$ lxc exec x1792905 -- dpkg -S /lib/modules
dpkg-query: no path found matching pattern /lib/modules


## enable proposed
$ echo "deb http://archive.ubuntu.com/ubuntu xenial-proposed main" | lxc file push - x1792905/etc/apt/sources.list.d/proposed.list
$ lxc exec x1792905 apt-get update -q
Get:1 http://security.ubuntu.com/ubuntu xenial-security InRelease [107 kB]
Hit:2 http://archive.ubuntu.com/ubuntu xenial InRelease                        
...
Get:40 http://archive.ubuntu.com/ubuntu xenial-proposed/main amd64 Packages [63.2 kB]
Get:41 http://archive.ubuntu.com/ubuntu xenial-proposed/main Translation-en [26.4 kB]
Fetched 25.9 MB in 6s (3980 kB/s)                                              
Reading package lists... Done

## install new package.
$ lxc exec x1792905 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 20 not upgraded.
Need to get 4380 B of archives.
After this operation, 2048 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu xenial-proposed/main amd64 cloud-initramfs-copymods all 0.27ubuntu1.6 [4380 B]
Fetched 4380 B in 0s (53.3 kB/s)                   
(Reading database ... 25666 files and directories currently installed.)
Preparing to unpack .../cloud-initramfs-copymods_0.27ubuntu1.6_all.deb ...
Unpacking cloud-initramfs-copymods (0.27ubuntu1.6) over (0.27ubuntu1.5) ...
Setting up cloud-initramfs-copymods (0.27ubuntu1.6) ...
Processing triggers for initramfs-tools (0.122ubuntu8.12) ...


## And see what owns /lib/modules now.
$ lxc exec x1792905 -- dpkg -S /lib/modules
cloud-initramfs-copymods: /lib/modules

$ lxc exec x1792905 -- ls -l /lib/modules
total 0