Dockerfile: Fix wrong base image arch when cross-building with podman

[jschintag]

When cross-building the image for s390x on amd64, podman would use an amd64 image for the base of the final image.

What this PR does / why we need it:
Fix cross-building manually on amd64 with podman.

What works now:

On x86_64 host:

• docker amd64 build
• podman amd64 build
• docker s390x build
• podman s390x build (fixed by this PR)

On s390x host:

• podman s390x build

Special notes for your reviewer:

[jschintag]

With this we have the following for working and tested by me build ways:

On x86_64 host:

• docker amd64 build
• podman amd64 build
• docker s390x build
• podman s390x build (fixed by this PR)

On s390x host:

• podman s390x build

Since docker does not currently offer builds for s390x on their fedora mirror, i haven't tested it.

[oshoval]

Great, thanks

/lgtm
/approve

Nit:
in the above comment, i think you meant on the 4th bullet
"podman, host amd64, target s390x (fixed by this PR)"

best to add the sum of all what is supported and what isn't to the PR desc please

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[jschintag]

Nit: in the above comment, i think you meant on the 4th bullet "podman, host amd64, target s390x (fixed by this PR)"

best to add the sum of all what is supported and what isn't to the PR desc please

I had them grouped by host, but the formating made it not look that way... I fixed it and copied it to the PR description as well.