running modified containers with podman

Everybody (who runs containers) knows this situation: you've been running happycontainer:stable for a while and it's been great but now something external changed and you need to adjust the code while there is still no release with the patch.

I've encountered exactly this when our Home-Assistant stopped showing the presence of our cat correctly, but we've also been discussing this at work recently.

Now the most obvious (to me?) solution would be to build a new container, based on the original one, and perform the modifications at build time. Something like this:

FROM happycontainer:stable
RUN curl … | patch -p1

But that's not interactive, and if you don't have a patch readily available, that's not what you want. (And I'll save you the idea of RUNing sed and friends to alter files!)

You could run vim inside the container, but that requires vim to be installed there in the first place. And a reasonable configuration. And…

Well, turns out podman can mount the root fs of a running container.

[root@sai ~]# podman mount homeassistant
/var/lib/containers/storage/overlay/f3ac502d97b5681989dff

And if you're running as non-root, you'll get an error:

[container@sai ~]$ podman mount homeassistant
Error: cannot run command "podman mount" in rootless mode, must execute `podman unshare` first

Luckily the solution is in the error message - use podman unshare

[container@sai ~]$ podman unshare
[root@sai ~]# podman mount homeassistant
/home/container/.local/share/containers/storage/overlay/95d3809d53125e4d40ad05e52efaa5a48e6e61fe9b8a8478416ff44459c7eb31/merged

So in both cases (root and rootless) we get a path, which is the mounted root fs and we can edit things in there as we like.

[root@sai ~]# vi /home/container/.local/share/containers/storage/overlay/95d3809d53125e4d40ad05e52efaa5a48e6e61fe9b8a8478416ff44459c7eb31/merged/usr/src/homeassistant/homeassistant/components/surepetcare/binary_sensor.py

Once done, the container can be unmounted again, and the namespace left

[root@sai ~]# podman umount homeassistant
homeassistant
[root@sai ~]# exit
[container@sai ~]$

At this point we have modified the code inside the container, but the running process is still using the old code. If we restart the container now to restart the process, our changes will be lost.

Instead, we can commit the changes as a new layer and tag the result.

[container@sai ~]$ podman commit homeassistant docker.io/homeassistant/home-assistant:stable

And now, when we restart the container, it will use the new code with our changes 🎉

[container@sai ~]$ systemctl --user restart homeassistant

Is this the best workflow you can get? Probably not. Does it work? Hell yeah!