build Python and Django images for aarch64

Add configuration to build Python and Django base images for aarch64.

Example pipeline: https://gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/-/pipelines/379104.

A straightforward change which just builds our base images for aarch64 as well. We skip one image because the version of alpine used is too old to have a rust compiler in the arm64 image. Quite why Python packages feel the need to require a rust compiler to install is a different question, of course. The Django images are skipped because the grpcio package takes so long on arm64 that the CI jobs time out.

Closes #73 (closed)