travis: build appimage for linux

.travis.yml

@@ -70,6 +70,17 @@ jobs:
       script: ./contrib/osx/make_osx
       after_script: ls -lah dist && md5 dist/*
       after_success: true
+    - name: "AppImage build"
+      sudo: true
+      language: c
+      python: false
+      services:
+        - docker
+      install:
+        - sudo docker build --no-cache -t electrum-appimage-builder-img ./contrib/build-linux/appimage/
+      script:
+        - sudo docker run --name electrum-appimage-builder-cont -v $PWD:/opt/electrum --rm --workdir /opt/electrum/contrib/build-linux/appimage electrum-appimage-builder-img ./build.sh
+      after_success: true
     - stage: release check
       install:
           - git fetch --all --tags