Public keys not saved in repo for future updates

[ramou]

While probably a minor issue, the apt.gpg file used in the image does not seem to be permanently installed (or it has become outdated), so you get warnings about updating things when using the image later. The intent is for users of this image to use it as-is, but it's still not great that this is messy, so I'm adding this to the list of things I should fix when, by some miracle, I have spare time:

Singularity> apt update
Ign:1 http://ftp.ca.debian.org/debian stretch InRelease
Get:2 http://cran.rstudio.com/bin/linux/debian stretch-cran35/ InRelease [4,409 B]
Get:3 http://ftp.ca.debian.org/debian stretch Release [118 kB]
Get:4 http://ftp.ca.debian.org/debian stretch Release.gpg [3,177 B]
Err:2 http://cran.rstudio.com/bin/linux/debian stretch-cran35/ InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B8F25A8A73EACF41
Get:5 http://ftp.ca.debian.org/debian stretch/main amd64 Packages [7,080 kB]
Get:6 http://ftp.ca.debian.org/debian stretch/main Translation-en [5,377 kB]
Reading package lists... Done
W: GPG error: http://cran.rstudio.com/bin/linux/debian stretch-cran35/ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B8F25A8A73EACF41
E: The repository 'http://cran.rstudio.com/bin/linux/debian stretch-cran35/ InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

The desired behaviour would be for all the required public keys to be installed in the image and available as needed.