Add multi-arch integration test and fix docker-buildx cross-compile

[rvanlaak]

ARM images fail with exec /manager: exec format error, and there was no test coverage to catch this class of regression before publishing images.

See rvanlaak#1 where I've asked Copilot to help with properly testing multiarch.

Ran https://github.com/rvanlaak/sm-kubernetes/actions/runs/28868331949 to verify the added multiarch build and test work correctly. The build itself fails at the end of the successful build because my upstream repo - obviously - does not have push permissions on Bidwarden's Github container registry:

ERROR: failed to build: failed to solve: failed to push ghcr.io/bitwarden/sm-operator:dev: denied: permission_denied: The requested installation does not exist.

🎟️ Tracking

Fixes #131

📔 Objective

Test multiarch images, drop old Dockerfile.cross build approach.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ rvanlaak
❌ Copilot
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[bitwarden-bot]

Thank you for your contribution! We've added this to our internal tracking system for review.
ID: PM-40068
Link: https://bitwarden.atlassian.net/browse/PM-40068

Details on our contribution process can be found here: https://contributing.bitwarden.com/contributing/pull-requests/community-pr-process.