docs: list supported algorithms for ADD --checksum

[dvdksn]

Closes docker/docs#18682

[thaJeztah]

@crazy-max Are there still improvements to be made to the error message as well? (I haven't checked); see the original ticket;

I'd add to better document the error you get when the digest algorithm is unsupported. I tried ADD --checksum:md5 and the error I was getting from a docker compose build command was "unsupported digest algorithm", which IMO is not very clear. I didn't even know it was about the ADD command, and I was digging around the image digest for a good bit before thinking of looking at the ADD command.

[crazy-max]

@crazy-max Are there still improvements to be made to the error message as well? (I haven't checked); see the original ticket;

I'd add to better document the error you get when the digest algorithm is unsupported. I tried ADD --checksum:md5 and the error I was getting from a docker compose build command was "unsupported digest algorithm", which IMO is not very clear. I didn't even know it was about the ADD command, and I was digging around the image digest for a good bit before thinking of looking at the ADD command.

Good point we could improve the error message indeed