Whatever the outcome of bug 1565332, one thing is true; Debian 10 and Ubuntu 18.04 both could be somewhat larger than the existing Ubuntu 16.04 docker image.

For example:
Ubuntu 18.04: 1264.467 mb
Debian 10: 1392.757 mb
Ubuntu 16.04: I recall it being ~900 mb at some point, though it isn't 100% certain

Now, since Ubuntu 16.04 is the default image on CI, it is often cached so does not need to be downloaded then loaded into docker.

Even then, I suspect we can do better than the 1.2GB size of the newer images.

In this bug, aim for the following:

• reduce the size of the docker image, with overachieve goal being 50%
• eliminate as many support and configuration files as possible
• reduce the number of steps in the setup phase
• reduce the time taken to build, with overachieve goal being somewhere in the mid-20min mark

Through some clever streamlining, I was able to reduce the size of Ubuntu 18.04 docker image greatly.

Before

[taskcluster 2019-11-20 20:40:30.663Z] Downloading artifact "public/image.tar.zst" from task ID: bwK_x_HlSPGc8U8MtbLcQA.
[taskcluster 2019-11-20 20:40:35.664Z] Download Progress: 20.63%
[taskcluster 2019-11-20 20:40:40.665Z] Download Progress: 43.83%
[taskcluster 2019-11-20 20:40:45.665Z] Download Progress: 65.72%
[taskcluster 2019-11-20 20:40:50.665Z] Download Progress: 86.41%
[taskcluster 2019-11-20 20:40:55.445Z] Downloaded artifact successfully.
[taskcluster 2019-11-20 20:40:55.445Z] Downloaded 1264.381 mb
[taskcluster 2019-11-20 20:40:55.445Z] Decompressing downloaded image
[taskcluster 2019-11-20 20:41:14.236Z] Loading docker image from downloaded archive.
[taskcluster 2019-11-20 20:44:25.525Z] Image 'public/image.tar.zst' from task 'bwK_x_HlSPGc8U8MtbLcQA' loaded.  Using image ID sha256:493ea02d08a09b7cc4023bc0d1e3d9ef3464e83750512604e79a0c338e14adfa.
[taskcluster 2019-11-20 20:44:25.877Z] === Task Starting ===

After

[taskcluster 2019-11-20 22:20:49.993Z] Downloading artifact "public/image.tar.zst" from task ID: LtyijhkdQ_aoMRjMLd5niQ.
[taskcluster 2019-11-20 22:20:54.995Z] Download Progress: 34.08%
[taskcluster 2019-11-20 22:20:59.994Z] Download Progress: 79.06%
[taskcluster 2019-11-20 22:21:02.301Z] Downloaded artifact successfully.
[taskcluster 2019-11-20 22:21:02.301Z] Downloaded 736.608 mb
[taskcluster 2019-11-20 22:21:02.302Z] Decompressing downloaded image
[taskcluster 2019-11-20 22:21:13.654Z] Loading docker image from downloaded archive.
[taskcluster 2019-11-20 22:22:32.625Z] Image 'public/image.tar.zst' from task 'LtyijhkdQ_aoMRjMLd5niQ' loaded.  Using image ID sha256:3420c675887b3751601d04c501fe5c93df064e2e2689dfea80eb8d9036b7f3b5.

For reference, the pushes:
before: https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&selectedJob=277271951&revision=e391131f9afe2a1c82e33d11f0aafc452dd7106e
after: https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=f4386735652d3e20d2709c32b66775bfab07dcd2

Image build time has been reduced from 35+ minutes to 17 minutes.

For the initial patch I have reduced the scope of work, since the changes were becoming quite large and variables untenable.

Specifically, the big gains produced by the --no-install-recommends require some careful analysis that may end up costing some weeks, so I have decided to remove that from the scope of part 1 patch.

The potential gains are large, leading to half the size of any typical package installation (eg. python-pip), as shown below:
default:

Need to get 99.8 MB of archives.
After this operation, 346 MB of additional disk space will be used.

with --no-install-recommends:

Need to get 8613 kB of archives.
After this operation, 27.3 MB of additional disk space will be used.

But this caused numerous problems with browser-chrome, xpcshell and other test suites.

https://hg.mozilla.org/mozilla-central/rev/5ba72034381f

First part of the streamlining has landed, I will investigate other opportunities to save some costs for mozilla in further rounds.

Renamed to optimize docker image.

https://hg.mozilla.org/mozilla-central/rev/851ae2bf3a56

https://hg.mozilla.org/mozilla-central/rev/29efea1762a8

Closing as this chapter of work is done.

I think this is meant to be a 'stderr into stdout' redirection

https://hg.mozilla.org/mozilla-central/rev/b5790b779d9c