Got this to work eventually. If anybody is interested, basically if you're using an apple silicon machine, you need to force the build to use linux/amd64.
Basically, you need to add that to main dockerfile and the storage service dockerfile, ie FROM --platform=linux/amd64 ubuntu:${UBUNTU_VERSION} AS base-builder
And in the docker-compose file, mysql, gearmand, fits, and clamavd need to have added platform: linux/amd64
-Joe