App Platform Ruby Buildpack v1.244.3 error about no Gemfile.lock

I’m trying to deploy a Jekyll app on App Engine. However, even though I have a Gemfile committed in the repo, it gives this error:

[2023-08-15 05:49:42] ╭──────────── git repo clone ───────────╼
[2023-08-15 05:49:42] │  › fetching app source code
[2023-08-15 05:49:43] │ => Selecting branch "master"
[2023-08-15 05:49:44] │ => Checking out commit "8bbf222a736f486bf36c68d59f1c66361886477a"
[2023-08-15 05:49:44] │ 
[2023-08-15 05:49:44] │  ✔ cloned repo to /workspace
[2023-08-15 05:49:45] ╰────────────────────────────────────────╼
[2023-08-15 05:49:45] 
[2023-08-15 05:49:45]  › configuring custom build command to be run at the end of the build:
[2023-08-15 05:49:45]    │ bundle; bundle exec jekyll build
[2023-08-15 05:49:45] 
[2023-08-15 05:49:45] ╭──────────── buildpack detection ───────────╼
[2023-08-15 05:49:45] │  › using Ubuntu 18.04 stack
[2023-08-15 05:49:47] │ Detected the following buildpacks suitable to build your app:
[2023-08-15 05:49:47] │ 
[2023-08-15 05:49:47] │    heroku/ruby                  v1.244.3  (Ruby)
[2023-08-15 05:49:47] │    digitalocean/ruby-appdetect  v0.0.3    
[2023-08-15 05:49:47] │    digitalocean/procfile        v0.0.4    (Procfile)
[2023-08-15 05:49:47] │    digitalocean/custom          v0.1.2    (Custom Build Command)
[2023-08-15 05:49:47] │ 
[2023-08-15 05:49:47] │ For documentation on the buildpacks used to build your app, please see:
[2023-08-15 05:49:47] │ 
[2023-08-15 05:49:47] │    Ruby  v1.244.3  https://do.co/apps-buildpack-ruby
[2023-08-15 05:49:47] ╰─────────────────────────────────────────────╼
[2023-08-15 05:49:47] 
[2023-08-15 05:49:47] ╭──────────── app build ───────────╼
[2023-08-15 05:49:49] │ -----> Compiling Ruby/NoLockfile
[2023-08-15 05:49:49] │ 
[2023-08-15 05:49:49] │  !
[2023-08-15 05:49:49] │  !     No such file or directory @ rb_sysopen - ./Gemfile.lock
[2023-08-15 05:49:49] │  !
[2023-08-15 05:49:49] │ /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/helpers/bundler_wrapper.rb:200:in `read': No such file or directory @ rb_sysopen - ./Gemfile.lock (Errno::ENOENT)
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/helpers/bundler_wrapper.rb:200:in `read'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/helpers/bundler_wrapper.rb:200:in `major_bundler_version'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/helpers/bundler_wrapper.rb:213:in `detect_bundler_version_and_dir_name!'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/helpers/bundler_wrapper.rb:76:in `initialize'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/ruby.rb:32:in `new'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/ruby.rb:32:in `bundler'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/ruby.rb:36:in `bundler'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/ruby.rb:248:in `ruby_version'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/ruby.rb:230:in `slug_vendor_ruby'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/ruby.rb:123:in `build'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/bin/support/ruby_build:25:in `block in <main>'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/lib/language_pack/base.rb:175:in `log'
[2023-08-15 05:49:49] │         from /cnb/buildpacks/heroku_ruby/1.244.3/bin/support/ruby_build:24:in `<main>'
[2023-08-15 05:49:49] │ ERROR: failed to build: exit status 1
[2023-08-15 05:49:49] │ 
[2023-08-15 05:49:49] │ 
[2023-08-15 05:49:49] │ For documentation on the buildpacks used to build your app, please see:
[2023-08-15 05:49:49] │ 
[2023-08-15 05:49:49] │    Ruby  v1.244.3  https://do.co/apps-buildpack-ruby
[2023-08-15 05:49:49] │ 
[2023-08-15 05:49:49] │  ✘ build failed
[]

My repo is public at joshingyou/jekyll-lens: A Jekyll theme for photographers and photo bloggers. 📸 (github.com)

The custom build command I used is bundle; bundle exec jekyll build