Add Alpine (musl) build environment alongside Debian 13 and Rocky Linux 9

Mirrors the same pinned third-party library build sequence as debian13/
on Alpine 3.24, targeting musl libc, with the fixes needed to get the
whole non-Meta stack building cleanly:

- Adjusts the package install step for apk; builds makeself from a
  pinned source tag since it isn't packaged for stable Alpine releases;
  adds coreutils so GNU realpath (needed by liburing's configure script)
  is available in place of busybox's; drops sha256 checksum verification
  on curl-fetched archives.
- CMake on 3.24 is 4.x, which removed compatibility with
  cmake_minimum_required() versions below 3.5. Adds
  -DCMAKE_POLICY_VERSION_MINIMUM=3.5 (CMake's own sanctioned workaround,
  not a source patch) to every cmake configure call so unmigrated
  CMakeLists.txt in our pinned dependencies still configure.
- GCC on 3.24 is 15, which defaults to C23; C23 redefines an empty `()`
  in a function declaration to mean "no parameters" instead of the old
  K&R "unspecified parameters", which breaks a decades-old conftest in
  GMP 6.3.0's configure. Pins GMP's build to -std=gnu17 to restore the
  old semantics for that one dependency.
- Switches the GitHub mirror from gh-proxy.com (which started rejecting
  some repos with a content-filter 403 in this environment) to the
  project's own reverse proxy at wangguofang.net/github.com/.

Folly/Fizz/Wangle/mvfst/fbthrift/CacheLib are left commented out: their
portability layer reaches for glibc-only macros/symbols in multiple
places with no musl branch (off64_t/lseek64/pread64 in
folly/portability/Unistd.h, __ELF_NATIVE_CLASS in
folly/debugging/symbolizer/Elf.cpp, and likely more further in), and
patching each one as it surfaces risks introducing subtle bugs in code
we don't own. The reasoning is recorded inline for anyone revisiting
this later.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WGpSs9pKTbvPkmFvBaWhBp

alpine3.24/build.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+set -eu
+
+IMAGE_NAME="${IMAGE_NAME:-alpine3.24-source-stack}"
+SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+
+if [ -n "${CONTAINER_ENGINE:-}" ]; then
+    ENGINE=$CONTAINER_ENGINE
+elif command -v docker >/dev/null 2>&1; then
+    ENGINE=docker
+elif command -v podman >/dev/null 2>&1; then
+    ENGINE=podman
+else
+    echo "Error: Docker or Podman is required." >&2
+    exit 1
+fi
+
+echo "Building image $IMAGE_NAME with $ENGINE ..."
+exec "$ENGINE" build --tag "$IMAGE_NAME" "$SCRIPT_DIR"