[R-sig-Debian] Docker build issue

[Dave Lange]

Count me as a docker newbie building a debian python and R mytools
container. My first post. This was working a couple of weeks ago. My last
successful build was a R 4.01 release when the R:base docker hub project
used to list a number of base images and I used sections of a posted docker
file in that area. Then the install directions at Cran:R listed buster:4.01
instructions that I also added to my docker file. I decided to delete my
container and refresh over the weekend and my R install has been throwing
fits ever since. It looks like the cran:r and docker hub sites have
changed. It looks like a file in the debian buster-cran40 repository still
has a label for debian unstable.

Looking for answers!

Dave Lange
Kenmore, WA

last steps of the build log:
## Get:5 http://cloud.r-project.org/bin/linux/debian buster-cran40/
InRelease [4363 B]
## Get:6 http://deb.debian.org/debian buster/main amd64 Packages [7905 kB]
## Get:7 http://deb.debian.org/debian buster-updates/main amd64 Packages
[7868 B]
## Get:8 http://cloud.r-project.org/bin/linux/debian buster-cran40/
Packages [37.8 kB]
## Fetched 8398 kB in 3s (3116 kB/s)
## Reading package lists...
## Reading package lists...

The docker build command throws this error in red:
E: The value 'unstable' is invalid for APT::Default-Release as such a
release is not available in the sources

My dockerfile:
>From python:3

## Python docker commands not shown and not throwing errors

RUN apt-get update \
&& apt-get install -y --no-install-recommends \
ed \
less \
locales \
vim-tiny \
wget \
ca-certificates \
fonts-texgyre \
&& rm -rf /var/lib/apt/lists/*

RUN echo "deb http://cloud.r-project.org/bin/linux/debian buster-cran40/" >
/etc/apt/sources.list
##        && echo 'APT::Default-Release "stable";' >
/etc/apt/apt.conf.d/default

ENV R_BASE_VERSION 4.0.1

## Now install R and littler, and create a link for littler in
/usr/local/bin
RUN apt-get update \
&& apt-get install -t buster-cran40 r-base \
  && install.r docopt \
&& rm -rf /tmp/downloaded_packages/ /tmp/*.rds \
&& rm -rf /var/lib/apt/lists/*

CMD python

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