lint: Add missing docker.io prefix to ci/lint_imagefile #30501

Currently, the ci/lint_imagefile may pick the wrong (non-native) architecture due to the missing prefix.

For example, assuming the user has previously pulled an s390x image:

$ podman run --rm 'docker.io/s390x/debian:bookworm' dpkg --print-architecture
exec /usr/bin/dpkg: exec format error

Now, debian:bookworm will refer to the same image:

$ podman run --rm 'debian:bookworm' dpkg --print-architecture
exec /usr/bin/dpkg: exec format error

However, docker.io/debian:bookworm works fine:

 $ podman run --rm 'docker.io/debian:bookworm' dpkg --print-architecture
arm64

(Also includes a nit-fix from #30499 (review))

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage

For detailed information about the code coverage, see the test coverage report.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process.

If your review is incorrectly listed, please react with 👎 to this comment and the bot will ignore it on the next update.

utACK fa7bee13bf745d8d244fa8d3579a21016a0cb66d

Thanks!

ACK fa7bee13bf745d8d244fa8d3579a21016a0cb66d.