more automation

[hairyhenderson]

One of the more manual parts of releasing was grabbing checksums and copy/pasting them into the stackbrew-config.yaml file. This PR automates that - the getChecksums.sh is essentially a script I had locally but never committed it.

[francislavoie]

• Should we .gitignore the checksums.txt? I don't think we need to commit that.
• Should maybe link directly to https://github.com/docker-library/official-images/blob/master/library/caddy in the README to save a couple clicks, I'd probably make a PR via GitHub UI :pencil2: instead of checking out the official-images repo so a quick link is nice.
• Maybe put make gen-dockerfiles in the README as the first step instead of just make to avoid the whole "toss away the library/caddy changes" part?
• If it were me I'd maybe call them make gen and make lib or something shorter names, and make sure make lib doesn't also run gen-dokerfiles again redundantly :man_shrugging: