feat: Add musl support

[ptondereau]

Description

• We're adding a Docker build specific to musl, as the embedded PHP lib also needs to be compiled for musl, and the official images only have embedded SAPI for ZTS Alpine.
• Rust doctests are disabled because rustdoc --test has its own compilation pipeline that overrides some flags with -static-pie
• We're also re-exporting the runtime and static features for the whole project.
• embed tests are disabled for now.

Note: The Dockerfile could be improved by a lot. I did my job here, as I'm not a Docker expert. 😅

[coveralls]

Pull Request Test Coverage Report for Build 18750142719

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 27.709%

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Totals
Change from base Build 18747498499:	0.0%
Covered Lines:	1156
Relevant Lines:	4172

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💛 - Coveralls

[ptondereau]

waiting #566 to be merged first

[coveralls]

Pull Request Test Coverage Report for Build 19147027407

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 31.098%

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Totals
Change from base Build 19135394459:	0.0%
Covered Lines:	1359
Relevant Lines:	4370

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💛 - Coveralls