fix(Dockerfile, Makefile, manifest): Address user/group ID mapping and attempt to resolve ninja errors

Makefile

@@ -6,8 +6,13 @@ TARGET_DIR := ${PWD}/target
 OUTPUT_FILE := ${TARGET_DIR}/aseprite.flatpak
 BUILD_DIR := ${PWD}/build
 SRC_DIR := ${PWD}/src
-FLATPAK_STATE_DIR := ${PWD}/flatpak-state
-FLATPAK_CACHE_DIR := ${PWD}/flatpak-cache
+
+# Host-side cache directories for Flatpak builder and ccache
+# These will be mounted into the container
+FLATPAK_BUILDER_CACHE_DIR := ${PWD}/.flatpak-builder-cache
+FLATPAK_STATE_DIR := ${FLATPAK_BUILDER_CACHE_DIR}/state
+FLATPAK_CACHE_DIR := ${FLATPAK_BUILDER_CACHE_DIR}/cache
+CCACHE_DIR := ${PWD}/.ccache
 
 # Get current user and group IDs to avoid permission issues with Docker
 USER_ID := $(shell id -u)
@@ -21,52 +26,53 @@ all: build
 # Use .PHONY to ensure it always runs if called directly
 .PHONY: prepare-sources
 prepare-sources:
-	@echo "--- Preparing sources using prepare_sources.sh ---"
-	./prepare_sources.sh
-	./prepare_sources.sh --check-integrity
+@echo "--- Preparing sources using prepare_sources.sh ---"
+./prepare_sources.sh
+./prepare_sources.sh --check-integrity
 
 # Build the Docker image which runs flatpak-builder inside
 # Implicitly depends on sources being ready after prepare-sources runs
 .PHONY: build-image
 build-image:
-	@echo "--- Building Aseprite Flatpak builder image (${IMAGE_NAME}) ---"
-	@docker build -t ${IMAGE_NAME} -f ${DOCKERFILE} .
+@echo "--- Building Aseprite Flatpak builder image (${IMAGE_NAME}) ---"
+@docker build \
+--build-arg UID=${USER_ID} \
+--build-arg GID=${GROUP_ID} \
+-t ${IMAGE_NAME} -f ${DOCKERFILE} .
 
 # Run flatpak-builder inside a privileged container with proper user permissions
 .PHONY: run-flatpak-builder
 run-flatpak-builder:
-	@echo "--- Running flatpak-builder inside container ---"
-	@mkdir -p ${BUILD_DIR} ${FLATPAK_STATE_DIR} ${FLATPAK_CACHE_DIR}
-	@docker run \
-	--rm \
-	--privileged \
-	--device /dev/fuse \
-	-v ${SRC_DIR}:/sources:ro \
-	-v ${BUILD_DIR}:/build \
-	-v ${FLATPAK_STATE_DIR}:/root/.local/share/flatpak-builder \
-	-v ${FLATPAK_CACHE_DIR}:/root/.cache/flatpak-builder \
-	-v ${PWD}/com.aseprite.Aseprite.yaml:/build/com.aseprite.Aseprite.yaml:ro \
-	-e FLATPAK_USER_DIR=/build/.flatpak \
-	-e USER_ID=${USER_ID} \
-	-e GROUP_ID=${GROUP_ID} \
-	-w /build \
-	${IMAGE_NAME} \
-	sh -c "flatpak-builder --disable-rofiles-fuse --state-dir=/root/.local/share/flatpak-builder --ccache build-dir com.aseprite.Aseprite.yaml --repo=repo && if [ -d 'build-dir/files/third_party/externals/emsdk' ]; then rm -rf build-dir/files/third_party/externals/emsdk; fi && chown -R ${USER_ID}:${GROUP_ID} /build"
+@echo "--- Running flatpak-builder inside container ---"
+@mkdir -p ${BUILD_DIR} ${FLATPAK_STATE_DIR} ${FLATPAK_CACHE_DIR} ${CCACHE_DIR}
+@docker run \
+--rm \
+--privileged \
+--device /dev/fuse \
+-u ${USER_ID}:${GROUP_ID} \
+-v ${SRC_DIR}:/sources:ro \
+-v ${BUILD_DIR}:/build \
+-v ${FLATPAK_STATE_DIR}:/home/builder/.local/share/flatpak-builder \
+-v ${FLATPAK_CACHE_DIR}:/home/builder/.cache/flatpak-builder \
+-v ${CCACHE_DIR}:/home/builder/.cache/ccache \
+-v ${PWD}/com.aseprite.Aseprite.yaml:/build/com.aseprite.Aseprite.yaml:ro \
+-w /build \
+${IMAGE_NAME} \
+sh -c "flatpak-builder --disable-rofiles-fuse --state-dir=/home/builder/.local/share/flatpak-builder --ccache build-dir com.aseprite.Aseprite.yaml --repo=repo && if [ -d 'build-dir/files/third_party/externals/emsdk' ]; then rm -rf build-dir/files/third_party/externals/emsdk; fi"
 
 # Create the Flatpak bundle from the repo built in the previous step
 .PHONY: create-bundle
 create-bundle: run-flatpak-builder
-	@echo "--- Creating Flatpak bundle ---"
-	@docker run \
-	--rm \
-	--privileged \
-	--device /dev/fuse \
-	-v ${BUILD_DIR}:/build \
-	-e USER_ID=${USER_ID} \
-	-e GROUP_ID=${GROUP_ID} \
-	-w /build \
-	${IMAGE_NAME} \
-	sh -c "flatpak build-bundle repo aseprite.flatpak com.aseprite.Aseprite && chown ${USER_ID}:${GROUP_ID} aseprite.flatpak"
+@echo "--- Creating Flatpak bundle ---"
+@docker run \
+--rm \
+--privileged \
+--device /dev/fuse \
+-u ${USER_ID}:${GROUP_ID} \
+-v ${BUILD_DIR}:/build \
+-w /build \
+${IMAGE_NAME} \
+sh -c "flatpak build-bundle repo aseprite.flatpak com.aseprite.Aseprite"
 
 # Copy the final bundle from the intermediate directory to the output directory
 .PHONY: extract-flatpak