Add 'ADD --checksum' support

[ppettina]

This PR adds support for the --checksum flag in the ADD instruction:
https://docs.docker.com/engine/reference/builder/#verifying-a-remote-file-checksum-add---checksumchecksum-http-src-dest

Note, I know nothing about the Haskell programming language - I have written this by looking at this other PR #84. So please be kind!

Final goal would be to add another rule to Hadolint recommending the use of ADD --checksum with remote sources.

[ppettina]

Ping @lorenzo :)

[mkesper]

Does anyone here know how to work around this until this issue gets fixed? This does not seem like an error that can be ignored with inline comments in the Dockerfile.

[ppettina]

Does anyone here know how to work around this until this issue gets fixed? This does not seem like an error that can be ignored with inline comments in the Dockerfile.

We forked the hadolint repository internally, then put the content of this repo (language-docker) with this PR merged in vendored/language-docker (a full copy), then rebuilt as per step 2 in https://github.com/hadolint/hadolint?tab=readme-ov-file#setup.

[lorenzo]

thanks!