build: Added AppImage builder (#355)

* Docker files to create an AppImage

* Using ENTRYPOINT is a bit nicer here

* typo

* put with other dist files

dist/AppImage/build.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+# Set the TAG environment variable to build a specific tag
+
+# Make sure we're in the same direcotry as this script
+pushd $(dirname "$(realpath "$0")")
+
+if [ -z "$TAG" ]; then
+    docker build -t imhex-appimage-build .
+else
+    docker build --build-arg=TAG=$TAG -t imhex-appimage-build-$TAG .
+fi
+
+popd