"build images build all" fails for tutor v13.3.2 due to apt update failure (probably due to Debian repo change)

I am trying to do a step-by step upgrade. I am trying to get off Lilac and onto Maple. I installed tutor 13.3.2 which is the latest version for Maple. When I do tutor images build all I get the following error, which is probably not tutor’s fault, but rather a dependency that has changed. How can I work around it?

Building image docker.io/overhangio/openedx-forum:13.0.0
docker build -t docker.io/overhangio/openedx-forum:13.0.0 /home/ubuntu/.local/share/tutor/env/plugins/forum/build/forum
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            Install the buildx component to build images with BuildKit:
            https://docs.docker.com/go/buildx/

Sending build context to Docker daemon  5.632kB
Step 1/29 : FROM docker.io/ruby:2.5.7-slim-stretch
2.5.7-slim-stretch: Pulling from library/ruby
48839397421a: Pull complete 
e766eb7a99c4: Pull complete 
1473a499abfc: Pull complete 
cf6b4a7156a7: Pull complete 
2d68c964e3a1: Pull complete 
Digest: sha256:a501a713048e19cd1dd06e283fdd817675a53b2c57a7e0e36a4b14a139489bec
Status: Downloaded newer image for ruby:2.5.7-slim-stretch
 ---> 3c374af8fd62
Step 2/29 : MAINTAINER Overhang.IO <contact@overhang.io>
 ---> Running in 796f74507bc7
Removing intermediate container 796f74507bc7
 ---> e7d2717d5a2c
Step 3/29 : ENV DEBIAN_FRONTEND=noninteractive
 ---> Running in 57d84375be63
Removing intermediate container 57d84375be63
 ---> 3667f71a8911
Step 4/29 : RUN apt update &&   apt upgrade -y &&   apt install -y git wget autoconf bison build-essential libssl-dev libyaml-dev libreadline6-dev zlib1g-dev libncurses5-dev libffi-dev libgdbm-dev
 ---> Running in 1d2a351c31b5

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Ign:1 http://security.debian.org/debian-security stretch/updates InRelease
Err:2 http://security.debian.org/debian-security stretch/updates Release
  404  Not Found [IP: 151.101.2.132 80]
Ign:3 http://deb.debian.org/debian stretch InRelease
Ign:4 http://deb.debian.org/debian stretch-updates InRelease
Err:5 http://deb.debian.org/debian stretch Release
  404  Not Found
Err:6 http://deb.debian.org/debian stretch-updates Release
  404  Not Found
Reading package lists...
E: The repository 'http://security.debian.org/debian-security stretch/updates Release' does not have a Release file.
E: The repository 'http://deb.debian.org/debian stretch Release' does not have a Release file.
E: The repository 'http://deb.debian.org/debian stretch-updates Release' does not have a Release file.
The command '/bin/sh -c apt update &&   apt upgrade -y &&   apt install -y git wget autoconf bison build-essential libssl-dev libyaml-dev libreadline6-dev zlib1g-dev libncurses5-dev libffi-dev libgdbm-dev' returned a non-zero code: 100
Error: Command failed with status 100: docker build -t docker.io/overhangio/openedx-forum:13.0.0 /home/ubuntu/.local/share/tutor/env/plugins/forum/build/forum

(Possible workaround: I just disabled the forum plugin (because I’m not going to stop at Maple, but keep going up in revisions) and build proceeded. But I figured it was important to surface this bug too, as the container may need an update or something.)

Debian ended its support of Stretch last year: Debian | endoflife.date We are unable to provide a Debian mirror for outdated Stretch packages. This is one of the reasons why we are unable to support past releases. I’m afraid that if you want to keep running v13, you will have to patch your images yourself.