Draft: Pull existing images in the build step to reduce build time

An experiment to speed up build times.

So far:

• can't naively pull images since the multi-stage build ensures that the most time-consuming parts (installing dependencies) aren't in the final image

Next steps to investigate:

• add a new app stage with poetry, then install poetry packages from there
• build up to the poetry stage and then push that to the registry as an intermediate step for all the other images to build on