httpgd package not currently on CRAN

[hturner]

The {httpgd} package was archived on 2025-04-23. Potentially it will get back on CRAN at a later date, but in the meantime we should update the Dockerfile to prefer r-universe over CRAN, so that we install the development version of the package.

This also has the advantage of installing from a Linux binary rather than from source, but also means we need to update the Dockerfile to use ubuntu latest (see also #200, #213) as binaries are only built for the latest version of ubuntu.

[hturner]

Fixed by #235.