Proposal: Update 'zig build get-v8' instructions in README and Dockerfile

[Yamashou]

Currently, the get-v8 subcommand does not exist in the latest build.zig, but the README and Dockerfile still reference the legacy zig build get-v8 command. The actual command is now prepare-v8, so these references should be corrected.

Background

• As of PR upgrade build.zig to zig 0.11 #4 (upgrade build.zig to zig 0.11 #4) in September 2023, get-v8 did exist.
• Subsequent updates refactored build subcommands; the equivalent is now prepare-v8.
• However, the documentation (README.md, Dockerfile, etc.) still refers to the old zig build get-v8 command.

Proposal

• Update all mentions of zig build get-v8 in README.md and the Dockerfile to zig build prepare-v8.
• Review other documentation for similar corrections.

References

• Current build.zig
• PR #4: upgrade build. zig to zig 0.11 (when get-v8 existed)

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This update will help avoid confusion and ensure correct build instructions for users. Thank you for considering.