Container image builds fail due to older Poetry version

[apyrgio]

GitHub has switched the ubuntu-latest runner images we're using from Ubuntu 24.04 to Ubuntu 22.04 :

• Runner image on Oct 16th was Ubuntu 24.04: https://github.com/freedomofpress/dangerzone/actions/runs/11368463360/job/31623614028#step:1:8
• Runner image today is Ubuntu 22.04: https://github.com/freedomofpress/dangerzone/actions/runs/11375940576/job/31647550993#step:1:8

This looks like a bug by GitHub, probably happening now because we're getting closer to the October 30th deadline, when all runners who specify ubuntu-latest will use Ubuntu 24.04 by default. See https://github.blog/changelog/2024-09-25-actions-new-images-and-ubuntu-latest-changes/.

We can't do much here, other than explicitly specify ubuntu-24.04 as our runner image, and change it later on to ubuntu-latest.