[CI] Update to latest alpine linux container image

I'm not sure why we were using the `node` flavored ones.

Author: sbc100

.github/workflows/ci.yml

@@ -218,7 +218,7 @@ jobs:
     - name: test
       run: python check.py --binaryen-bin=out/bin
 
-  # Build with gcc 6.3 and run tests on Alpine Linux (inside chroot).
+  # Build with gcc and run tests on Alpine Linux (inside docker container).
   # Note: Alpine uses musl libc.
   # Keep in sync with build_release.yml
   build-alpine:
@@ -230,7 +230,7 @@ jobs:
         submodules: true
     - name: start docker
       run: |
-        docker run -w /src -dit --platform=linux/arm64 --name alpine -v $PWD:/src node:lts-alpine
+        docker run -w /src -dit --platform=linux/arm64 --name alpine -v $PWD:/src alpine:3.21
         echo 'docker exec alpine "$@";' > ./alpine.sh
         chmod +x ./alpine.sh
 

.github/workflows/create_release.yml

@@ -107,7 +107,7 @@ jobs:
           ${{ steps.archive-arm64.outputs.TARBALL }}
           ${{ steps.archive-arm64.outputs.SHASUM }}
 
-  # Build with gcc 6.3 and run tests on Alpine Linux (inside chroot).
+  # Build with gcc and run tests on Alpine Linux (inside docker container).
   # Note: Alpine uses musl libc.
   build-alpine:
     name: alpine
@@ -128,7 +128,7 @@ jobs:
         if [[ "${{ matrix.docker_platform }}" == "ubuntu-24.04-arm" ]]; then
           platform="--platform=linux/arm64"
         fi
-        docker run -w /src -dit $platform --name alpine -v $PWD:/src node:lts-alpine
+        docker run -w /src -dit $platform --name alpine -v $PWD:/src alpine:3.21
         echo 'docker exec alpine "$@";' > ./alpine.sh
         chmod +x ./alpine.sh