Simplify Dockerfiles and custom app guide (#714)

* 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

docker-bake.hcl

@@ -30,7 +30,7 @@ target "bench-test" {
 # Base for all other targets
 
 group "frappe" {
-    targets = ["frappe-worker", "frappe-nginx", "frappe-socketio"]
+    targets = ["frappe-worker", "frappe-nginx", "frappe-socketio", "assets-builder"]
 }
 
 group "erpnext" {
@@ -81,6 +81,13 @@ target "frappe-nginx" {
     tags =  tag("frappe-nginx", "${FRAPPE_VERSION}")
 }
 
+target "assets-builder" {
+  inherits = ["default-args"]
+  context = "images/nginx"
+  target = "assets_builder"
+  tags = tag("assets-builder", "${FRAPPE_VERSION}")
+}
+
 target "erpnext-nginx" {
     inherits = ["default-args"]
     context = "images/nginx"