FlowAuth base image deprecated

[jc-harrison]

The 'python3.8-alpine' tag of tiangolo/uwsgi-nginx-flask (the base docker image we use for FlowAuth) is no longer maintained. We should look at switching to a different one.

The 'python3.8' tag is still maintained, and would presumably be a drop-in replacement, but it is (unsurprisingly) a lot bigger than the alpine variant - I'm not sure whether that will be a problem.