Update ftl-build container to Alpine v3.21

[DL6ER]

What does this implement/fix?

Update ftl-build container to Alpine v3.21

HIGHLIGHTS:

• Linux kernel 6.12
• GCC 14
• LLVM (clang) 19

As always, the major upgrade to GCC 14.2.0 is the most important reason for us to upgrade our build environment.

As it turned out that quite a few changes were necessary to support Alpine 3.12, I decided it's better to pin the container to a certain version instead of the more generic latest. It can always be updated and we can use it to tag a new container when new Alpine versions are released. The PR with the necessary changes for FTL will be opened once this PR is merged and new containers are tagged to update the used build container at the same time in the FTL PR.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
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[DL6ER]

Patching is necessary due to a breaking change in GCC 14:

Implicit function declarations (-Werror=implicit-function-declaration)
It is no longer possible to call a function that has not been declared. In general, the solution is to include a header file with an appropriate function prototype. Note that GCC will perform further type checks based on the function prototype, which can reveal further type errors that require additional changes.

For well-known functions declared in standard headers, GCC provides fix-it hints with the appropriate #include directives:

error: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration]
   5 |   return strlen (s);
     |          ^~~~~~
note: include ‘<string.h>’ or provide a declaration of ‘strlen’
 +++ |+#include <string.h>
   1 |

On GNU systems, headers described in standards (such as the C standard, or POSIX) may require the definition of certain macros at the start of the compilation before all required function declarations are made available. See Feature Test Macros in the GNU C Library manual for details.

[yubiuser]

Maybe we should also port the monkey patch from
https://github.com/pi-hole/FTL/pull/2044/commits
for nicer BATS output

[DL6ER]

@yubiuser As TERM is now set by us in the tester, we get this:

Anything needed in addition? I don't think it needs to be "too pretty" as this isn't something we're looking often at.

[yubiuser]

I thought about this part

https://github.com/pi-hole/FTL/blob/18f91b0f09c651841a3c82f03d7790c4fd482fd6/.github/Dockerfile#L19-L22

[dschaper]

👍🏻