[R-sig-Debian] Update on docker Python:3 and adding R:4.x

[Dave Lange]

 I continue to receive an error installing R via dockerfile on a buster
image python:3.
E: The value 'buster-cran40' is invalid for APT::Default-Release as such a
release is not available in the sources

My starting point is the debian buster based Python:3 image adding a couple
of python specific configurations and then using the commands in the R
project documentation for installing R on Buster. I got slightly different
answers when I used apt versus apt-get. There were warnings about
unverified sources solved by a reference to the certificate key.

It turns out building the python container and commenting the R commands
out allowed me to manually step through my dockerfile lines. Its repeatable
that the R install fails with the error above when in the dockerfile.
Running the commands manually allows the installation to finish
successfully.  I sense that docker is multithreaded and hits the "use
buster-cran40" before it defines buster-cran40. Manually stepping through
the commands keeps the preferred order. At this point I'm happy with a
repeatable process.

It sounds like I have been re-inventing the wheel, which has been
educational for me. If someone wants to change my starting point to
something that already has stable/latest production for debian, Python3 and
R4 and will be updated for the future I would appreciate the head start.

Thanks for the volunteer support I have received here.
Dave Lange
Kenmore, WA

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