fix: make multi-architecture builds reliable

[jwcesign]

Why

Multi-platform builds downloaded dependencies separately for each architecture through a flaky proxy. A truncated wheel download caused pip install to fail immediately, and source changes invalidated the dependency layer because application files were copied first. The image also upgraded pip to an unpinned latest version on every build.

Proposed Changes

• Copy and install requirements.txt before application source so dependency layers remain reusable.
• Add a persistent BuildKit pip cache and bounded retries for interrupted package downloads.
• Remove the mutable, unnecessary pip self-upgrade.
• Add a minimal .dockerignore so only runtime source enters the build context.

Validation

• Fresh linux/amd64,linux/arm64 Buildx build passed with multiarch-proxy.
• Repeated dual-architecture build completed entirely from cache in 7.6 seconds.
• python -m unittest discover -s tests (6 tests)
• Pylint 10.00/10
• git diff --check

Release Note

NONE

[gitautomator]

Thanks to your contribution, the maintainers will review it as soon as they can!