Static site ignores environment_slug

Hi, I have a static site hosted on DO App Platform. I update it pretty rarely, but recently it’s started to fail at the build stage in a way that suggests it’s ignoring the appspec’s environment_slug.

I have this in my .do/app.yaml file (redacted name and repo):

name: XYZ
region: nyc
static_sites:
- build_command: bundle exec jekyll build -d ./public
  environment_slug: jekyll
  error_document: 404.html
  github:
    branch: main
    deploy_on_push: true
    repo: XYZ/XYZ
  name: XYZ
  source_dir: /

But the logs, included below, seem to suggest that it’s ignoring the environment_slug value of jekyll. Not sure why – I based it on this sample. My first thought was that maybe the jekyll environment had gone away; but using ruby shows the same. Also I cannot find any documentation for what values are possible here. The App Spec documentation just links to the App Platform documentation landing page.

What can I do to fix this? And what happened to make it stop working?

[2024-06-19 15:26:25]  › configuring custom build command to be run at the end of the build:
[2024-06-19 15:26:25]    │ bundle exec jekyll build -d ./public
[2024-06-19 15:26:25] 
[2024-06-19 15:26:25] ╭──────────── buildpack detection ───────────╼
[2024-06-19 15:26:26] │  › using Ubuntu 22.04 stack
[2024-06-19 15:26:26] │ Detected the following buildpacks suitable to build your app:
[2024-06-19 15:26:26] │ 
[2024-06-19 15:26:26] │    digitalocean/static  v0.0.3  (Static Assets)
[2024-06-19 15:26:26] │    digitalocean/custom  v0.1.2  (Custom Build Command)
[2024-06-19 15:26:26] ╰─────────────────────────────────────────────╼
[2024-06-19 15:26:26] 
[2024-06-19 15:26:26] ╭──────────── build caching ───────────╼
[2024-06-19 15:26:26] │  › checking for cache from a previous build
[2024-06-19 15:26:27] │ Previous image with name <registry-uri-0> not found
[2024-06-19 15:26:27] │ Layer cache not found
[2024-06-19 15:26:27] │ Layer cache not found
[2024-06-19 15:26:27] ╰───────────────────────────────────────╼
[2024-06-19 15:26:27] 
[2024-06-19 15:26:27] ╭──────────── app build ───────────╼
[2024-06-19 15:26:27] │ Running custom build command: bundle exec jekyll build -d ./public
[2024-06-19 15:26:27] │ bash: line 1: bundle: command not found
[2024-06-19 15:26:27] │ building: exit status 127
[2024-06-19 15:26:27] │ ERROR: failed to build: exit status 1
[2024-06-19 15:26:27] │ 
[2024-06-19 15:26:27] │ 
[2024-06-19 15:26:27] │  ✘ build failed