GHA: also build on Alpine to test LibreSSL which is used on OpenBSD

.github/workflows/alpine-bash.Dockerfile

@@ -0,0 +1,2 @@
+FROM alpine:3
+RUN ["apk", "--no-cache", "add", "bash"]

.github/workflows/linux.bash

@@ -1,12 +1,18 @@
 #!/bin/bash
 set -exo pipefail
 
-export PATH="/usr/lib/ccache:/usr/lib64/ccache:/opt/rh/devtoolset-11/root/usr/bin:$PATH"
+export PATH="/usr/lib/ccache/bin:/usr/lib/ccache:/usr/lib64/ccache:/opt/rh/devtoolset-11/root/usr/bin:$PATH"
 export CCACHE_DIR=/icinga2/ccache
 export CTEST_OUTPUT_ON_FAILURE=1
 CMAKE_OPTS=''
 
 case "$DISTRO" in
+  alpine:*)
+    apk add bison {boost,libressl}-dev ccache cmake flex g++ ninja-build tzdata
+    ln -vs /usr/lib/ninja-build/bin/ninja /usr/local/bin/ninja
+    CMAKE_OPTS="-DUSE_SYSTEMD=OFF $(echo -DICINGA2_WITH_{MYSQL,PGSQL,COMPAT,LIVESTATUS,PERFDATA,ICINGADB}=OFF)"
+    ;;
+
   amazonlinux:2)
     amazon-linux-extras install -y epel
     yum install -y bison ccache cmake3 gcc-c++ flex ninja-build \

.github/workflows/linux.yml

@@ -21,6 +21,7 @@ jobs:
       max-parallel: 2
       matrix:
         distro:
+          - alpine:bash # LibreSSL, used on OpenBSD
           - amazonlinux:2
           - amazonlinux:2023
           - centos:7 # and RHEL 7
@@ -51,7 +52,13 @@ jobs:
           path: ccache
           key: ccache/${{ matrix.distro }}
 
-      - name: Build
+      - name: Build Docker image
+        if: "matrix.distro == 'alpine:bash'"
+        run: >-
+          docker build --file .github/workflows/alpine-bash.Dockerfile
+          --tag alpine:bash `mktemp -d`
+
+      - name: Build Icinga
         run: >-
           docker run --rm -v "$(pwd):/icinga2" -e DISTRO=${{ matrix.distro }}
           ${{ matrix.distro }} /icinga2/.github/workflows/linux.bash