fix: container start test

[ryanwi]

Problem

The docker build load was failing which preventing the container test from having a container to test against. Docker doesn't support load with multiple architectures. This makes that more clear while still allowing for repos to publish images for multiple architectures.

Container test also would be preferred to happen after the container has had a chance to startup. But, maybe we need to support both and be more flexible?

Changes

• Allow container test to be executed after the container's default cmd has had time to run
• Only build single platform for load [1]
• git refs are for debugging/testing, can still demonstrate this as is. I will revert those once we're good with the other changes.

References:
[1] docker/buildx#59