non-multi-arch images now fail with clear error

[acarl005]

Description

This PR is a follow-up for this: https://warpdev.slack.com/archives/C0AC81SR0L9/p1771440212411899?thread_ts=1771437585.553369&cid=C0AC81SR0L9

The problem was that the image appeared to pull successfully but then failed with "image not found" immediately after.

The problem was actually we were missing an ARM build on the warpdotdev/dev-java:21 image and they were using an ARM worker. We do require image to have the same platform as the host in order to avoid virtualization overhead, but that requirement is unclear here.

This PR adds an explicit check and fails with a clearer error message in this case.

Note that I didn't catch this case during dogfooding b/c I only tested images with multi-arch images with modern manifest lists. For those images, we already get an error about that.

However, older non-multi-arch images will actually successfully pull even if you explictly request a different platform, see here: docker/for-mac#5625 (comment)

I tested this by creating a legacy non-multi-arch image, e.g.

# /tmp/test-amd64-only/Dockerfile
FROM --platform=linux/amd64 alpine:3.21
RUN echo "This image is intentionally amd64-only for testing platform mismatch detection."

docker buildx build --platform linux/amd64 --provenance=false --sbom=false --push -t warpdotdev/test-amd64-only:latest /tmp/test-amd64-only

I created an environment with that image...

❯ oz environment get 4rUUMF9bnTPNJug50i0qAz
Name: Test Alpine
Docker image: warpdotdev/test-amd64-only:latest
Repositories: None
Setup commands: None

...and tried running it with my MacBook as a self-hosted runner and I got the error I expected

20:18:38.000 ERR Task launch failed: taskID=a44eaf9b-2f77-4355-8c8e-da64fa27c69f, error=image warpdotdev/test-amd64-only:latest is for platform linux/amd64, but this worker requires linux/arm64

[bnavetta]

Nice find