When setting Python version to 3.10 using runtime.txt, you actually get v3.7

Bug description

Note: This bug was originally surfaced via a help request in gitter.

When you use a runtime.txt file to set the Python version, and request v3.10, you actually get version 3.7 (I suspect the default version r2d is configured to install?)

Expected behaviour

The Python version stated in the runtime.txt file is respected and Python version 3.10 is installed.

Actual behaviour

Instead of 3.10, Python 3.7 is available.

How to reproduce

I created an MWE repo to demonstrate this: https://github.com/sgibson91/pythonv3.10-binder-mwe

1. Launch the above repo in binder:
2. Open a Terminal
3. Execute the command python --version
4. See result is 3.7.*, not 3.10.*

Alternatively:

1. Open a new Notebook
2. Execute the below code in a cell:

import sys
print(sys.version)

1. See result is 3.7.*, not 3.10.*

Your personal set up

See files in https://github.com/sgibson91/pythonv3.10-binder-mwe for setup details.

[minrk]

As @manics mentioned, this was fixed very recently in repo2docker (#1175), but that update had yet to be deployed to mybinder.org due to https://github.com/jupyterhub/mybinder.org-deploy/issues/2337. That was just fixed and repo2docker updated by merging https://github.com/jupyterhub/mybinder.org-deploy/pull/2336, so the issue for 3.10 in particular should be resolved for both repo2docker and mybinder.org. But image caching means you’ll need to trigger a rebuild by pushing a new commit. This should work, I think: https://gke.mybinder.org/v2/gh/minrk/pythonv3.10-binder-mwe/HEAD

I think we can close this, as #552 already represents the issue of incorrect envs instead of errors for unsupported Python (fixed in #1184)

[sgibson91]

1. I installed r2d from main using python3 -m pip install https://github.com/jupyterhub/repo2docker/archive/main.zip (into a fresh conda env)
2. I locally cloned https://github.com/sgibson91/pythonv3.10-binder-mwe
3. I ran repo2docker . from the repo root
4. Logs from r2d:

Picked Local content provider.
Using local repo ..
Building conda environment for python=3.10
Using PythonBuildPack builder
Building conda environment for python=3.10
Building conda environment for python=3.10

(Just to note that I saw the same BuildPack logs highlighting v3.10 on Binder but it silently fell back to v3.7)

This time when I ran python --version, I got version 3.10. So this is a binder issue, not a r2d issue. Apologies for the noise! Shall I transfer this issue to the mybinder.org-deploy repo?