replace docker tags with corresponding digests

[gbrownmozilla]

This updates all of the Dockerfiles with FROM lines using constant tags. There remain several Dockerfiles that take an argument - those are left unchanged.

[escapewindow]

I wonder if we need something like python:3.8@sha256:037c262134bc0cffa81606421308eb0dbf5c851c2328ec585983f8c18553966f

[escapewindow]

INFO[0001] Invalid repository name (python:3.8), only [a-z0-9-_.] are allowed

Ah, I wonder if [our version of] kaniko doesn't allow for hash pinning, in which case we might not be able to do this.

[gbrownmozilla]

fwiw, that format is fine locally,:

$ docker run -t -i python:3.8@sha256:037c262134bc0cffa81606421308eb0dbf5c851c2328ec585983f8c18553966f
Python 3.8.12 (default, Sep 28 2021, 19:06:31) 
[GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 

[escapewindow]

Yeah. We use kaniko to build our docker images. I believe this is both more secure than running directly on docker, and avoids the whole docker-in-docker issue. But it doesn't support everything that docker supports.

[escapewindow]

Hm, https://github.com/GoogleContainerTools/kaniko/blob/0477900febfacb98b88abdbfaf8bbaeb05f51060/integration/dockerfiles/Dockerfile_test_volume_3#L4 looks promising.
I wonder if it's supported in a newer version of kaniko that we're not using yet.