build: add AppImage packaging pipeline and build scripts

packaging/appimage/build-appimage-docker.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+IMAGE_NAME="${APPIMAGE_DOCKER_IMAGE:-scanslicer-appimage-builder}"
+HOST_UID="${HOST_UID:-$(id -u)}"
+HOST_GID="${HOST_GID:-$(id -g)}"
+
+if ! command -v docker >/dev/null 2>&1; then
+    echo "Missing required command: docker" >&2
+    exit 1
+fi
+
+docker build \
+  -f "${ROOT_DIR}/packaging/appimage/Dockerfile" \
+  -t "${IMAGE_NAME}" \
+  "${ROOT_DIR}"
+
+docker run --rm \
+  -e HOST_UID="${HOST_UID}" \
+  -e HOST_GID="${HOST_GID}" \
+  -v "${ROOT_DIR}:/work" \
+  -w /work \
+  --entrypoint bash \
+  "${IMAGE_NAME}" \
+  -lc 'set -euo pipefail; bash packaging/appimage/build-appimage.sh; if [ -d /work/appimage ]; then chown -R "${HOST_UID}:${HOST_GID}" /work/appimage; fi'
+
+echo "Docker AppImage build complete. Output in ${ROOT_DIR}/appimage/dist"