ci: add alpine build to github ci

Problem: There is no automated build of flux-core on a musl
libc system, so PRs are not checked for portability to non-glibc
environments.

Add a ci build on alpine to the github workflow matrix. Only run a
quick check since there are still many fixes required for sharness
tests in this environment.

src/test/generate-matrix.py

@@ -270,6 +270,20 @@ def __str__(self):
     docker_tag=True,
 )
 
+matrix.add_build(
+    name="alpine - quick check",
+    image="alpine",
+    command_args="--quick-check",
+    args=(
+        "--prefix=/usr"
+        " --sysconfdir=/etc"
+        " --with-systemdsystemunitdir=/etc/systemd/system"
+        " --localstatedir=/var"
+        " --with-flux-security"
+    ),
+    docker_tag=True
+)
+
 # inception
 matrix.add_build(
     name="inception",