Added multi-arch support.

[barbun]

Issue

Database images are built with plain docker build with no --platform flag so the resulting image inherits the builder's native architecture — typically linux/amd64.

That causes two problems:

• Local development on Apple Silicon - pulling and running these images forces amd64 emulation, with a noticeable performance penalty for MariaDB/MySQL workloads.
• No opt-in multi-arch support - even though upstream base images publish multi-arch manifests, the task never produces a matching multi-arch output image.

Proposed solution

Build and push sanitised database images as multi-arch manifests using docker buildx.

[shreddedbacon]

This needs some investigation to whether an underlying docker-host that would build these images has access to QEMU to be able to build multi-arch.