[FEATURE] Disable default mirror for installation of R dependencies in docker

[kverstae]

Feature summary

When you currently install R dependencies in a container via the viash config, it uses the mirror "https://cran.rstudio.com" (set here)
It would be useful if we could disable this behavior (so using the default mirror) or provide a mirror ourselves.

Why is this feature beneficial?

When you create a container based on rocker/r-ver, you don't want to overwrite the mirror used during installation. These containers use a specific mirror where the packages are locked in a specific version. This makes them very reproducible, since install.packages will always install the same version.
When overwriting the mirror, you lose this reproducibility, since it will try to download the most recent version available instead from the cran.rstudio mirror.

Alternatives considered

It is currently possible to work around this issue, but the solution is not very elegant. You can just pass the install commands via the script requirement instead:

- type: r
     script: install.packages("remotes"); remotes::install_github("powellgenomicslab/DropletQC")

This works, but can become messy if you need to install a lot of dependencies...

Possible solution

Maybe repos = "https://cran.rstudio.com" can just be omitted from the install_... commands for R dependencies?

Confirmation

• I have searched the existing issues to make sure this is not a duplicate request.
• I have provided clear and concise information about the feature.