ci: update alpine to 3.21

[crazy-max]

No description provided.

[crazy-max]

https://github.com/tonistiigi/xx/actions/runs/12200741186/job/34037574118?pr=174#step:5:783

 2.509 not ok 4 cross
2.510 # (in test file test-apk.bats, line 40)
2.512 #   `[ -f "/$(xx-info)/lib/libz.so.1" ]' failed

Seems since 3.21 it has moved to /usr/lib: https://pkgs.alpinelinux.org/contents?file=libz.so.1&path=&name=&branch=edge&repo=&arch=

[crazy-max]

https://github.com/tonistiigi/xx/actions/runs/12201048147/job/34038587244#step:5:869

Looks like the arch has changed for i386:

 #85 55.68 # (from function `testHelloGO' in file test-go.bats, line 241,
#85 55.68 #  in test file test-go.bats, line 306)
#85 55.68 #   `testHelloGO' failed
#85 55.68 # file /tmp/a.out does not match expected target architecture 386: ELF 32-bit LSB executable, Intel i386, version 1 (SYSV), statically linked, Go BuildID=tlJHujLf8JLPAF5dYPfF/rijSNAaw8swSPvNg4IGt/4-GT7PqK9YJ69YYFBQrl/Hp7UJLaeCGcsmukHOQlY, with debug_info, not stripped

Expected Intel 80386 but got Intel i386.

[crazy-max]

These ones are not related, should be fixed in another PR.

[tonistiigi]

These ones are not related, should be fixed in another PR.

Agree, but we should fix that issue before merging this.

[tonistiigi]

should also add new cases to https://github.com/tonistiigi/xx/blob/master/src/test-verify.bats