change build behaviour to support dynamic package selection

scripts/apk/ci-package-cache.sh

@@ -111,7 +111,7 @@ registry_upgrade_packages() {
 }
 
 prepare() {
-  local selected
+  local build_matrix selected
   declare -A selected_map=()
 
   download_registry_metadata
@@ -138,7 +138,9 @@ prepare() {
     } >> "${GITHUB_ENV}"
   fi
   if [[ -n "${GITHUB_OUTPUT:-}" ]]; then
+    build_matrix="$({ printf '%s\n' "${ALPINE_PACKAGES}" | tr ' ' '\n' | jq -Rsc 'split("\n") | map(select(length > 0)) | {package: .}'; })"
     printf 'selected_packages=%s\n' "${ALPINE_PACKAGES}" >> "${GITHUB_OUTPUT}"
+    printf 'build_matrix=%s\n' "${build_matrix}" >> "${GITHUB_OUTPUT}"
     if [[ -n "${ALPINE_PACKAGES}" ]]; then
       printf 'packages_selected=true\n' >> "${GITHUB_OUTPUT}"
     else