Add files for building AppImage of Kleopatra with Docker
* Makefile.am (EXTRA_DIST): Add new files. * README: Add instructions for building the AppImage. * docker/appimage/Dockerfile, docker/appimage/build-appimage.sh, docker/build-appimage-docker-image.sh, docker/run-appimage-build.sh: New.
- GnuPG-bug-id: T5592