Official Docker image of Datasette does not support VirtualKNN2

[rdmurphy]

Hello! I hit this last night while getting my database of LA County's election results online. Back when I did this in 2022 everything worked fine, but it appears that if you have a reasonably up to date copy of libspatialite on your computer and use it to create a database (via sqlite-utils) it creates a database that expects to be used with a copy of the library that supports the new-ish KNN2 feature.

I ended up just writing my own Dockerfile to get this working (using an Alpine image), but I think the issue the official image is having is that Debian's stable version of libsqlite3-mod-spatialite is still on 5.0.* instead of 5.1.* (when I believe KNN2 was added). Debian's package repository does have a reference to a 5.1.* version, but it's in the unstable package repository.