Upgrade Alpine to 3.21 (see CVE-2024-9143)

[Commandcracker]

Description

Upgrade Alpine in Dockerfile to 3.21.
Fixes CVE-2024-9143⁠

Testing

Docker build and run

Checklist

Things need to be done before this Pull Request can be merged.

• Code is well-formatted and adheres to project style guidelines: cargo fmt
• Code does not produce any clippy warnings: cargo clippy
• All unit tests pass: cargo test
• I added new unit tests, so other people don't accidentally break my code by changing other parts of the codebase. How?

[Snowiiii]

Thanks @Commandcracker