AppImage packaging: containerized build, host-only py_func/lua_func

build.sh runs appimage-builder in a Debian Bookworm container (Podman or
Docker) so it works on Arch / non-Debian hosts. Uses single src/requirements.txt;
TESTIUM_VERSION exported in runtime.env.

package/appimage/AppImageBuilder.yml

@@ -24,9 +24,8 @@ AppDir:
 
     runtime:
         env:
-            SEQUENCER_REV: '{{APP_VERSION}}'
+            TESTIUM_VERSION: '{{APP_VERSION}}'
             PYTHONPATH: $APPDIR/usr/lib/python3.11/site-packages:$APPDIR/usr/lib/python3.11
-            QT_QPA_PLATFORM: xcb
 
         path_mappings:
             - /usr/share/matplotlib/mpl-data/matplotlibrc:$APPDIR/etc/matplotlibrc
@@ -69,12 +68,13 @@ AppDir:
 
         # Set python 3.11 as default
         ln -fs python3.11 $TARGET_APPDIR/usr/bin/python3
-        # Install pip
-        if [ ! -f "get-pip.py" ]; then curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py; fi
+
+        # Bootstrap pip into the AppDir Python
+        if [ ! -f "get-pip.py" ]; then curl -sS https://bootstrap.pypa.io/get-pip.py -o get-pip.py; fi
         python3.11 get-pip.py --break-system-packages
 
         # Install application dependencies in AppDir
-        python3.11 -m pip install --break-system-packages --upgrade --isolated --no-input --ignore-installed --prefix=$TARGET_APPDIR/usr -r requirements.txt
+        python3.11 -m pip install --break-system-packages --upgrade --isolated --no-input --ignore-installed --prefix=$TARGET_APPDIR/usr -r ../../src/requirements.txt
 
         export PIP_CONFIG_FILE=$HOME/.pip/pip.conf
         python3.11 -m pip install --break-system-packages --upgrade --isolated --no-input --ignore-installed --prefix=$TARGET_APPDIR/usr ../../src/dist/testium-{{APP_VERSION}}-py3-none-any.whl