Propagate architecture and os from base to target images

[briandealwis]

• Adds architecture and os fields to Image/ImageBuilder and ContainerConfigurationTemplate, defaulting to current amd64/linux
• Modifies ImageToJsonTranslator and JsonToImageTranslator to propagate architecture/os
• Modifies BuildImageStep to propagate architecture/os from the base image to the built image
• Adds some explicit tests, and modifies existing tests to us wasm/js to ensure we're not just picking up the default amd64/linux
• Verified by hand by building against image s390x/openjdk:11-stretch

Note that this PR does not expose any facilities for changing the arch/os. I'll leave that to a separate PR.

Fixes #1547

[TadCordle]

nit: Sets (same for setOs javadoc)

[chanseokoh]

Note that this PR does not expose any facilities for changing the arch/os. I'll leave that to a separate PR.

I don't think there is a need to change the arch/os. If they wanted a different arch/os, they should use the image with the arch/os they want. For example, arm64v8/openjdk or winmad64/openjdk.

Now that said, I think now this PR undermines reproducibility, just a bit. That is, when the base image is openjdk, Jib will build an AMD image on AMD and an ARM image on ARM. We need to think about this.

[chanseokoh]

Jib will build an AMD image on AMD and an ARM image on ARM.

Actually, I'm not sure if this is the case with Jib. OTOH, I believe Docker build automatically pulls in the matching os/arch.

[briandealwis]

I don't think it undermines reproducibility: we propagate the os/arch from the base image. Docker Hub doesn't silently remap between architectures, so we should get the same base image for the same image ref.

I can totally get that docker build may silently swap os/arch. I don't see how it can work otherwise since it will be executing binaries for the current system.

But we should document the difference.

[chanseokoh]

I don't think it undermines reproducibility: we propagate the os/arch from the base image. Docker Hub doesn't silently remap between architectures, so we should get the same base image for the same image ref.

Ah, OK. That makes sense.