this will make it much easier to debug problems for instances that run
unreleased versions!
when run on a tagged commit, `git describe --tags` prints the tag name;
otherwise it prints something like `2025.4.3-32-ga4c0ef824c` which
means:
- the closest tag is 2025.4.3
- there are 32 commits between that tag and this commit
- this commit's id is `a4c0ef824c` (the `g` is just a prefix)
notice that the version as reported by the frontend (in
`/about-sharkey` for example) is _not_ changed, that one is still
sourced from `/package.json` (so, for example, you don't get a
"sharkey has been updated!" pop-up every time)
872258b04c