feat: allow an optional platform definition for container arch

[chickenandpork]

This enhancement (with accompanying example) shows how to set a list of platforms for the os/arch of the docker env so that the go_test can be executed in a controlled platform. For example, this allows a macOS/arm64 build to trigger a linux/amd64 test container.

The example is duplicated and extended from the existing go-test-image-test to preserve that example, and to show what changes when the pathname of the test changes (which some additional documentation).

I was unable, however, to create a functional example with a second built image in the docker-compose: the functionality of local_image_targets and data attributes just didn't cause the install script from the oci_load() to be present, earning no install script present.

fixes #32

[salesforce-cla]

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[salesforce-cla]

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[chickenandpork]

(@kriscfoster waiting on the CLA-bot to catch up... -- iterated emails to find one on my account)

[chickenandpork]

(pushed a different email address to get the CLA workflow to restart)

[chickenandpork]

@kriscfoster I see exec format error in what I can see from the logs. That's an Ubuntu -- linux/amd64 then?

I don't see this failure, but I'm on a darwin/arm64, maybe for some reason the platform is preferring am arm64, so it doesn't hurt me but hurts th x86_64/amd64 arch ? I'm guessing a bit :(

Maybe we build a multi-arch-manifest image, with arch matching the container, and let the host docker decide which architecture fits? (such would be another multi_platform() wrap and an oci_index() )

[kriscfoster]

@kriscfoster I see exec format error in what I can see from the logs. That's an Ubuntu -- linux/amd64 then?

I don't see this failure, but I'm on a darwin/arm64, maybe for some reason the platform is preferring am arm64, so it doesn't hurt me but hurts th x86_64/amd64 arch ? I'm guessing a bit :(

Maybe we build a multi-arch-manifest image, with arch matching the container, and let the host docker decide which architecture fits? (such would be another multi_platform() wrap and an oci_index() )

Yeah, that sounds possible. I'm not so familiar with this issue so unfortunately don't have a quick answer :(
However, I can probably take a look in a few days (today is just a little busy).