CI: build on Alpine Linux

Add a CI step to build on Alpine Linux, to avoid future breakage on
systems running musl libc.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

.github/workflows/ci.yml

@@ -3,8 +3,8 @@ name: ci
 on: [pull_request]
 
 jobs:
-  build:
-    name: build
+  build_ubuntu:
+    name: build on Ubuntu
     runs-on: ubuntu-22.04
     steps:
       - name: checking out
@@ -15,3 +15,13 @@ jobs:
         working-directory: tgt
         run: |
           make
+  build_alpine:
+    name: build on Alpine Linux
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Setup Alpine Linux
+        uses: jirutka/setup-alpine@v1
+
+      - name: build
+        run: |
+          make