ci: pin alpine to 3.18 or earlier

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

@@ -324,10 +324,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["3.0", "3.1", "3.2", "3.3-rc"]
+        - { ruby: "3.0", flavor: "alpine" }
+        - { ruby: "3.1", flavor: "alpine3.18" }
+        - { ruby: "3.2", flavor: "alpine3.18" }
+        - { ruby: "3.3-rc", flavor: "alpine3.18" }
     runs-on: ubuntu-latest
     container:
-      image: ruby:${{matrix.ruby}}-alpine
+      image: ruby:${{matrix.ruby}}-${{matrix.flavor}}
     steps:
       - uses: actions/checkout@v4
       - uses: actions/download-artifact@v4