I think I'm going to go with directories to match each milestone.

So for M70 we'll have:
deb https://storage.googleapis.com/cros-packages/70 stretch main

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/tremplin/+/c266fb63b94664686649231932b4c9951b4f01f7

commit c266fb63b94664686649231932b4c9951b4f01f7
Author: Stephen Barber <smbarber@chromium.org>
Date: Tue Aug 28 23:17:52 2018

tremplin: use CrOS milestone in the guest

If the guest is Debian, set the milestone version in cros.list for apt.

Add a bind mount to allow guests to determine which CrOS milestone
it is being run on.

BUG= chromium:829239 , chromium:870529 
TEST=cat /dev/.cros_milestone from container
TEST=cat /etc/apt/sources.list.d/cros.list

Change-Id: Ic43ea5118f2c63d8b2e1a343230a2511c48cddbd
Reviewed-on: https://chromium-review.googlesource.com/1171557
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>

[modify] https://crrev.com/c266fb63b94664686649231932b4c9951b4f01f7/src/chromiumos/tremplin/main.go
[add] https://crrev.com/c266fb63b94664686649231932b4c9951b4f01f7/src/chromiumos/tremplin/version.go
[modify] https://crrev.com/c266fb63b94664686649231932b4c9951b4f01f7/src/chromiumos/tremplin/tremplin.go

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/tremplin/+/28019a2c5518434c11c1e5a4dfdd8542b1937a61

commit 28019a2c5518434c11c1e5a4dfdd8542b1937a61
Author: Stephen Barber <smbarber@chromium.org>
Date: Sat Sep 01 06:42:54 2018

tremplin: allow milestone format string in image server

BUG= chromium:870529 
TEST=manual with cicerone_client

Change-Id: I6bb634961d0b05e51f1b11479a64dffc90f1aa00
Reviewed-on: https://chromium-review.googlesource.com/1196213
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>

[modify] https://crrev.com/28019a2c5518434c11c1e5a4dfdd8542b1937a61/src/chromiumos/tremplin/tremplin.go

Your change meets the bar and is auto-approved for M70. Please go ahead and merge the CL to branch 3538 manually. Please contact milestone owner if you have questions.
Owners: benmason@(Android), kariahda@(iOS), geohsu@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/tremplin/+/f3041ca65a32021a1c1c6c36149b24c36b043440

commit f3041ca65a32021a1c1c6c36149b24c36b043440
Author: Stephen Barber <smbarber@chromium.org>
Date: Sun Sep 02 20:51:19 2018

tremplin: allow milestone format string in image server

BUG= chromium:870529 
TEST=manual with cicerone_client

Change-Id: I6bb634961d0b05e51f1b11479a64dffc90f1aa00
Reviewed-on: https://chromium-review.googlesource.com/1196213
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
(cherry picked from commit 28019a2c5518434c11c1e5a4dfdd8542b1937a61)
Reviewed-on: https://chromium-review.googlesource.com/1201542
Reviewed-by: Stephen Barber <smbarber@chromium.org>
Commit-Queue: Stephen Barber <smbarber@chromium.org>

[modify] https://crrev.com/f3041ca65a32021a1c1c6c36149b24c36b043440/src/chromiumos/tremplin/tremplin.go

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/tast-tests/+/9bc323d98ce2032d2591d68efb04e03efbe9f1de

commit 9bc323d98ce2032d2591d68efb04e03efbe9f1de
Author: Stephen Barber <smbarber@chromium.org>
Date: Fri Sep 07 14:17:52 2018

tast-tests: vm: use milestone for component/container

BUG= chromium:870529 
TEST=vm.StartTerminaVM

Change-Id: Ibf28a89a536b61a60a4a4a248dd10d7c64e1187c
Reviewed-on: https://chromium-review.googlesource.com/1194321
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>

[modify] https://crrev.com/9bc323d98ce2032d2591d68efb04e03efbe9f1de/src/chromiumos/tast/local/vm/vm.go
[modify] https://crrev.com/9bc323d98ce2032d2591d68efb04e03efbe9f1de/src/chromiumos/tast/local/vm/util.go

This fix prevents sudo apt-add-repository from working because cros.list is not writable now.

please file a new bug if you're seeing a problem

With this fix, what will happen in non-Debian distros?  Will https://storage.googleapis.com/cros-packages always have the latest cros packages?

We can roll discussion of this into  issue 883616  where I was going to track removing the .list file from the Debian package.

Verified on M70 and M71.