Fix strip choking on non-binary files; bump Alpine to 3.23 and implement multi-stage build

[biliwala]

Issue:
The Alpine Docker build fails with a strip: ss-setup: file format not recognized error. This occurs because the strip command targets all files in /usr/local/bin, including non-binary scripts such as ss-nat and ss-setup.

Solution:
The build logic has been updated to use scanelf to identify and target only ELF binaries for stripping. This replaces the manual file exclusion method (grep -v) with an automated format check.

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Improvements:

• Bump base image Alpine to 3.23
• Implement multi-stage build to reduce image size

[madeye]

Thanks — the scanelf -E ET_DYN -E ET_EXEC approach is the more robust fix for strip choking on the shell scripts (ss-nat/ss-setup). Note this overlaps with #3031, which edits the same strip line (and also bumps Alpine 3.16→3.19). I'd like to land both behaviors together; could you rebase so the scanelf strip and the Alpine bump don't conflict? Keeping open pending a Docker image build check.

[biliwala]

Rebased on vndroid/master.
By the way, I also added a multi-stage build here. It reduces the disk usage from 28.2MB to 22.8MB (content size from 7.29MB to 6.24MB).
Let me know if you want me to keep it here or split the multi-stage build into a separate PR later.
Ready for the build check.