fix: pin base image to alpine:3.21 to avoid GCC 15 build failures

alpine:latest now resolves to GCC 15.2.0 which causes two separate
failures when building Node.js v24.5.0:
- riscv64: GCC 15 internal compiler error (segfault) in V8's
  thread-id.cc after ~5 hours of build time
- ppc64le: GCC 15 treats highway's ppc_vsx-inl.h template-body
  diagnostics as errors, failing at ~9 minutes

Pin to alpine:3.21 (GCC 14.2.0) to restore stable builds across all
architectures.

Signed-off-by: Bruno Verachten <gounthar@gmail.com>

Author: gounthar

staticnode/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:latest
+FROM alpine:3.21
 ARG VERSION
 # One of x64 or arm64 or ppc64le or riscv64
 ARG ARCH