CI: Add Alpine Linux 3.23 runner to the pipeline (#18087)

Add an Alpine Linux 3.23 runner to the CI chain to run OpenZFS builds
and tests against musl libc.

Currently, zfs_send_sparse is killed after 10 minutes on Alpine, causing
cascading EBUSY failures in the test suite. With zfs_send_sparse
disabled, the ZFS test suite reaches a pass rate of 94.62%.

This commit introduces the required Alpine-specific setup and a small
set of shell and cloud-init compatibility fixes that also apply to
existing Linux runners.

The Alpine runner is not enabled by default and is not executed for new
pull requests.

Sponsored-by: ERNW Research GmbH - https://ernw-research.de/

Signed-off-by: Alexander Moch <amoch@ernw.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>

.github/workflows/scripts/qemu-2-start.sh

@@ -43,6 +43,12 @@ case "$OS" in
     OSv="almalinux9"
     URL="https://repo.almalinux.org/almalinux/10/cloud/x86_64/images/AlmaLinux-10-GenericCloud-latest.x86_64.qcow2"
     ;;
+  alpine3-23)
+    OSNAME="Alpine Linux 3.23.2"
+    # Alpine Linux v3.22 and v3.23 are unknown to osinfo as of 2025-12-26.
+    OSv="alpinelinux3.21"
+    URL="https://dl-cdn.alpinelinux.org/alpine/v3.23/releases/cloud/generic_alpine-3.23.2-x86_64-bios-cloudinit-r0.qcow2"
+    ;;
   archlinux)
     OSNAME="Archlinux"
     URL="https://geo.mirror.pkgbuild.com/images/latest/Arch-Linux-x86_64-cloudimg.qcow2"
@@ -216,13 +222,21 @@ if [ ${OS:0:7} != "freebsd" ]; then
 hostname: $OS
 
 users:
-- name: root
-  shell: $BASH
-- name: zfs
-  sudo: ALL=(ALL) NOPASSWD:ALL
-  shell: $BASH
-  ssh_authorized_keys:
-    - $PUBKEY
+  - name: root
+    shell: /bin/bash
+    sudo: ['ALL=(ALL) NOPASSWD:ALL']
+  - name: zfs
+    shell: /bin/bash
+    sudo: ['ALL=(ALL) NOPASSWD:ALL']
+    ssh_authorized_keys:
+      - $PUBKEY
+    # Workaround for Alpine Linux.
+    lock_passwd: false
+    passwd: '*'
+
+packages:
+  - sudo
+  - bash
 
 growpart:
   mode: auto
@@ -305,3 +319,23 @@ else
   scp ~/src.txz "root@vm0:/tmp/src.txz"
   ssh root@vm0 'tar -C / -zxf /tmp/src.txz'
 fi
+
+#
+# Config for Alpine Linux similar to FreeBSD.
+#
+if [ ${OS:0:6} == "alpine" ]; then
+  while pidof /usr/bin/qemu-system-x86_64 >/dev/null; do
+    ssh 2>/dev/null zfs@vm0 "uname -a" && break
+  done
+  # Enable community and testing repositories.
+  ssh zfs@vm0 "sudo rm -rf /etc/apk/repositories"
+  ssh zfs@vm0 "sudo setup-apkrepos -c1"
+  ssh zfs@vm0 "echo '@testing http://dl-cdn.alpinelinux.org/alpine/edge/testing' | sudo tee -a /etc/apk/repositories"
+  # Upgrade to edge or latest-stable.
+  #ssh zfs@vm0 "sudo sed -i 's#/v[0-9]\+\.[0-9]\+/#/edge/#g' /etc/apk/repositories"
+  #ssh zfs@vm0 "sudo sed -i 's#/v[0-9]\+\.[0-9]\+/#/latest-stable/#g' /etc/apk/repositories"
+  # Update and upgrade after repository setup.
+  ssh zfs@vm0 "sudo apk update"
+  ssh zfs@vm0 "sudo apk add --upgrade apk-tools"
+  ssh zfs@vm0 "sudo apk upgrade --available"
+fi