* Remove git reference from install-app
Also fixed issue with missing sites/assets folder
* Update custom app guide according to changes
* Ignore apps that have no frontend code instead of failing
* Add assets builder image
* Use assets builder in custom_app tutorial
* Use erpnext in custom app tutorial
* Add info about base images (frappe or erpnext)
* Add assets-builder image to frappe group so it is built in CI
* Update backend image:
- Fix mounted caching
- Uncomplicate ERPNext build
- Fix root-frappe permissions
* Add build-assets script for simpler frontend build
* Add install-app script for backend build
* Rename build-assets to install-app for frontend build
* Update custom app builds according to new main dockerfiles
* Cache pip packages in custom app example backend dockerfile
* Update custom app guide
* Fix typo in backend dockerfile
* Add info about install-app scripts in readme
