build: Simplify appimage build (#356)

* Simplify creating an AppImage

- package.sh packages the build result into an AppImage. It requires
squashfs-tools to work
- runtime-x86_64 is a binary distributed by the AppImage project that takes
care of extracting and running the image

* use AppRun from AppImage project

* clean up no longer needed bits

* Keep docker way of working around

- Docker now also uses `package.sh`, so no more need for FUSE.
- Fetch binaries instead of storing in git.

* Fix details

* wait a little longer for the container to start

dist/AppImage/Dockerfile

@@ -2,6 +2,7 @@ FROM debian:bullseye-slim
 LABEL maintainer Example <example@example.com>
 
 ARG TAG=master
+ARG REPO=https://github.com/WerWolv/ImHex.git
 
 USER root
 
@@ -13,13 +14,12 @@ RUN apt-get update \
         git \
         cmake \
         curl \
-        libfuse2 \
-        file
+        squashfs-tools
 
 # Fetch source and dependencies
 RUN mkdir -p /source \
     && cd /source \
-    && git clone https://github.com/WerWolv/ImHex.git \
+    && git clone $REPO \
     && cd ImHex \
     && git checkout $TAG \
     && git submodule update --init --recursive \
@@ -35,27 +35,6 @@ RUN mkdir -p /source/ImHex/build \
     && make -j
 
 # Prepare for AppImage
-RUN mkdir -p /source/ImHex.AppDir/usr/bin \
-    && mkdir -p /source/ImHex.AppDir/usr/lib \
-    && mkdir -p /source/ImHex.AppDir/usr/share/imhex/plugins \
-    && cp /source/ImHex/build/imhex /source/ImHex.AppDir/usr/bin/imhex \
-    && cp /source/ImHex/build/plugins/builtin/builtin.hexplug /source/ImHex.AppDir/usr/share/imhex/plugins
-
-COPY AppRun ImHex.desktop imhex.png /source/ImHex.AppDir/
-#RUN inkscape -z -o /source/ImHex.AppDir/imhex.png -w 128 -h 128 /source/ImHex/res/icon.svg
-
-# Gather the needed libraries
-RUN chmod a+x /source/ImHex.AppDir/AppRun \
-    && ldd /source/ImHex/build/imhex | awk '/ => /{print $3}' | xargs -I '{}' cp '{}' /source/ImHex.AppDir/usr/lib
-
-# Package the prepared AppDir
-RUN cd /source \
-    && curl -L https://github.com/AppImage/AppImageKit/releases/download/13/appimagetool-x86_64.AppImage -o appimagetool-x86_64.AppImage \
-    && chmod a+x appimagetool-x86_64.AppImage
-
-# All that remains is the following. However, due to docker limitations with fuse
-# this cannot be done in a build container.
-#
-# /source/appimagetool-x86_64.AppImage ImHex.AppDir
-
-ENTRYPOINT sh -c '/source/appimagetool-x86_64.AppImage /source/ImHex.AppDir; sleep 60'
+RUN cd /source/ImHex/dist/AppImage \
+    && ./package.sh /source/ImHex/build \
+    && mv /source/ImHex/build/ImHex-x86_64.AppImage /