As per  issue 870529  comment 13, the bind mount appears to break the add-apt-repository tool.

Debian packages are often frustratingly out of date.

Here is a great blog post showing how to get a functioning penguin under Ubuntu: https://linuxiumcomau.blogspot.com/2018/08/introduction-to-crostini-part-3-using.html

In the blog post he does:
echo "deb https://storage.googleapis.com/cros-packages stretch main" > /etc/apt/sources.list.d/cros.list
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1397BC53640DB551

I'm guessing this fix won't break that but wondering what version of cros tools will be available at https://storage.googleapis.com/cros-packages since the URL isn't versioned.

The cros-packages URL will be locked to M69's packages. Each subsequent version gets a subdirectory to match the CrOS milestone (cros-packages/70, cros-packages/71, ...).

I would really /not/ recommend using our apt repos for anything other than Debian stretch right now. We don't have any testing of these packages on other distros, so there's a high likelihood of broken deps eventually creeping in.

Hoping Ubuntu will be supported in future...

Looking at my debian penguin on CrOs 70, cros.list doesn't appear to have been bind-mounted:

$ cat /etc/apt/sources.list.d/cros.list 
deb https://storage.googleapis.com/cros-packages stretch main

This penguin was created before moving to CrOs 70 in stable.  Shouldn't it read:

deb https://storage.googleapis.com/cros-packages/70 stretch main

We didn't have any breaking guest changes in M70, so containers created on M69 will continue to use that repo until M71.

The bind mounts will also be going away, and instead the tremplin daemon will overwrite cros.list on container startup.

 Issue 889098  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/tremplin/+/97641e724229dd199a9f5242482e73730eb76080

commit 97641e724229dd199a9f5242482e73730eb76080
Author: Stephen Barber <smbarber@chromium.org>
Date: Wed Sep 26 03:41:04 2018

tremplin: write apt repo list instead of bind-mounting

Also add newline to end of cros.list template.

BUG= chromium:883616 
TEST=cros.list is writable from container

Change-Id: Ie95e6ab8d6cf9195347e3f26ac7d2f4f68a7768b
Reviewed-on: https://chromium-review.googlesource.com/1237934
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>

[modify] https://crrev.com/97641e724229dd199a9f5242482e73730eb76080/src/chromiumos/tremplin/version.go
[modify] https://crrev.com/97641e724229dd199a9f5242482e73730eb76080/src/chromiumos/tremplin/tremplin.go

Verified on component 11101.0.0.