Add Alpine APK packaging to CI build

Add fpm-based Alpine APK packaging alongside existing DEB/RPM/Pacman
packages. Alpine APKs use `linux` in the filename to distinguish from
OpenWrt APKs which use the `openwrt` prefix.

.github/build_alpine_apk.sh

@@ -0,0 +1,81 @@
+#!/usr/bin/env bash
+
+set -e -o pipefail
+
+ARCHITECTURE="$1"
+VERSION="$2"
+BINARY_PATH="$3"
+OUTPUT_PATH="$4"
+
+if [ -z "$ARCHITECTURE" ] || [ -z "$VERSION" ] || [ -z "$BINARY_PATH" ] || [ -z "$OUTPUT_PATH" ]; then
+  echo "Usage: $0 <architecture> <version> <binary_path> <output_path>"
+  exit 1
+fi
+
+PROJECT=$(cd "$(dirname "$0")/.."; pwd)
+
+# Convert version to APK format:
+#   1.13.0-beta.8  -> 1.13.0_beta8-r0
+#   1.13.0-rc.3    -> 1.13.0_rc3-r0
+#   1.13.0         -> 1.13.0-r0
+APK_VERSION=$(echo "$VERSION" | sed -E 's/-([a-z]+)\.([0-9]+)/_\1\2/')
+APK_VERSION="${APK_VERSION}-r0"
+
+ROOT_DIR=$(mktemp -d)
+trap 'rm -rf "$ROOT_DIR"' EXIT
+
+# Binary
+install -Dm755 "$BINARY_PATH" "$ROOT_DIR/usr/bin/sing-box"
+
+# Config files
+install -Dm644 "$PROJECT/release/config/config.json" "$ROOT_DIR/etc/sing-box/config.json"
+install -Dm755 "$PROJECT/release/config/sing-box.initd" "$ROOT_DIR/etc/init.d/sing-box"
+install -Dm644 "$PROJECT/release/config/sing-box.confd" "$ROOT_DIR/etc/conf.d/sing-box"
+
+# Service files
+install -Dm644 "$PROJECT/release/config/sing-box.service" "$ROOT_DIR/usr/lib/systemd/system/sing-box.service"
+install -Dm644 "$PROJECT/release/config/sing-box@.service" "$ROOT_DIR/usr/lib/systemd/system/sing-box@.service"
+
+# Completions
+install -Dm644 "$PROJECT/release/completions/sing-box.bash" "$ROOT_DIR/usr/share/bash-completion/completions/sing-box.bash"
+install -Dm644 "$PROJECT/release/completions/sing-box.fish" "$ROOT_DIR/usr/share/fish/vendor_completions.d/sing-box.fish"
+install -Dm644 "$PROJECT/release/completions/sing-box.zsh" "$ROOT_DIR/usr/share/zsh/site-functions/_sing-box"
+
+# License
+install -Dm644 "$PROJECT/LICENSE" "$ROOT_DIR/usr/share/licenses/sing-box/LICENSE"
+
+# APK metadata
+PACKAGES_DIR="$ROOT_DIR/lib/apk/packages"
+mkdir -p "$PACKAGES_DIR"
+
+# .conffiles
+cat > "$PACKAGES_DIR/.conffiles" <<'EOF'
+/etc/conf.d/sing-box
+/etc/init.d/sing-box
+/etc/sing-box/config.json
+EOF
+
+# .conffiles_static (sha256 checksums)
+while IFS= read -r conffile; do
+  sha256=$(sha256sum "$ROOT_DIR$conffile" | cut -d' ' -f1)
+  echo "$conffile $sha256"
+done < "$PACKAGES_DIR/.conffiles" > "$PACKAGES_DIR/.conffiles_static"
+
+# .list (all files, excluding lib/apk/packages/ metadata)
+(cd "$ROOT_DIR" && find . -type f -o -type l) \
+  | sed 's|^\./|/|' \
+  | grep -v '^/lib/apk/packages/' \
+  | sort > "$PACKAGES_DIR/.list"
+
+# Build APK
+apk mkpkg \
+  --info "name:sing-box" \
+  --info "version:${APK_VERSION}" \
+  --info "description:The universal proxy platform." \
+  --info "arch:${ARCHITECTURE}" \
+  --info "license:GPL-3.0-or-later with name use or association addition" \
+  --info "origin:sing-box" \
+  --info "url:https://sing-box.sagernet.org/" \
+  --info "maintainer:nekohasekai <contact-git@sekai.icu>" \
+  --files "$ROOT_DIR" \
+  --output "$OUTPUT_PATH"