Add support for GITHUB_PAT handling with private repo dependencies

[yogat3ch]

Hi @VincentGuyader & @ColinFay, This PR will eventually fix #18. It currently only modifies dock_from_desc to add handling of the GITHUB_PAT as a build-arg to enable fetching of private Github repos during docker build. A comment is included in the Dockerfile reminding the user to use the --build-arg GITHUB_PAT=[github PAT] flag when running docker build. There is an info message also indicating this fact which also informs the user that using this method causes the GITHUB_PAT to be exposed in the image metadata and thus the image must be kept private if uploaded to Docker Hub.

I would appreciate feedback on this approach thus far before I implement a similar method for handling private repos on the dock_from_renv function.

Can y'all let me know if this is satisfactory?

[yogat3ch]

Just added a couple more features:

• Added support for specifying a sha256 hash of the rocker version to control the architecture of the image. Related to golem#885
• Added support for excluding the dependencies specified in Suggests in docker_from_desc for smaller docker images.
• Alphabetizes the dependencies so it's easier to locate dependencies in the Dockerfile

[michkam89]

Hello there, I'm just curious if this is going to be merged? I'm thinking about working on #43 and I could use this code :)

[yogat3ch]

@michkam89 , tagging @VincentGuyader here to get some eyes on this

[yogat3ch]

Hey @statnmap, thanks for the review here! I haven't had a chance to implement the changes yet but I'll have some time off over the holidays where I can hopefully get to it!

[yogat3ch]

Hey @statnmap , I had an opportunity to document the sha256 parameter in the preceding commits. Is there anything else?

[yogat3ch]

Hey @statnmap, I think a safer way to do this is to copy an .Renviron file to the Docker image temporarily with the GITHUB_PAT set therein, and then delete it after renv::restore is run in the build process with RUN rm .Renviron cmd in the Dockerfile. This avoids exposing the GITHUB_PAT` in the run log when the image is uploaded to Docker Hub. Should I implement this instead?