bundle exec rake: command not found on Ubuntu 20.04 docker host - bundler

Question

I have a rails app (using the ruby:2.6-alpine base image) in Docker with an entrypoint executing some "bundle exec rake" calls. It runs fine in Ubunto on WSL2 and did run fine in Docker on an Ubuntu 20.04 host until recently. Now it fails on the Ubuntu Server with
bundler: command not found: rake
Install missing gem executables with `bundle install`
Entering the docker container and executing bundle check gives the following result:
The Gemfile's dependencies are satisfied
Executing bundle install results in:
Using rake 13.0.1
Using concurrent-ruby 1.1.8
Using i18n 1.8.9
...
Using webpacker 5.2.1
Bundle complete! 113 Gemfile dependencies, 175 gems now installed.
As you can see, rake is installed, there is nothing to install.
Executing bundle exec rake from the same docker image in Ubuntu on WSL2 it just works as expected.
How can I find out why calling bundle exec rake fails with bundler: command not found: rake in this environment?
Edit
When I create my image with ruby:2.6.7-alpine, it works on both hosts (Ubuntu as well as Ubuntu on WSL2), but with ruby:2.6.8-alpine I get the error on the Ubuntu server while it still works on the WSL2 Ubuntu. That explains why it worked fine until recently: before I used the ruby:2.6-alpine label, which was updated to ruby:2.6.8-alpine recently.