This updates the base image to python:3.12.6-slim. Rebuilding the image seems to fix issues with apt-get that appeared again some time after #97, and it seemed good to go ahead and update the Python version to make a new release.

Here's an internal test job using the tag from this branch, showing that apt-get update works. For comparison, here's a job using the 8.0.0 tag.

• (For Civis employees only) Reference to a relevant ticket in the pull request title
• Changelog entry added to CHANGELOG.md at the repo's root level
• Description of change in the pull request description
• If applicable, unit tests have been added and/or updated
• The CircleCI builds have all passed