ci: pin alpine to 3.18

because 3.19 fails due to #434. While I work on better musl support,
this makes CI a bit more meaningful.

Author: flavorjones

.github/workflows/gem-install.yml

@@ -327,7 +327,7 @@ jobs:
         ruby: ["3.0", "3.1", "3.2", "3.3-rc"]
     runs-on: ubuntu-latest
     container:
-      image: ruby:${{matrix.ruby}}-alpine
+      image: ruby:${{matrix.ruby}}-alpine3.18
     steps:
       - uses: actions/checkout@v4
       - uses: actions/download-artifact@v4