feat: allow layered custom image build (#1497)

speed up build time
pull builder image
pack built assets into base image

images/production/Containerfile

@@ -70,7 +70,7 @@ RUN useradd -ms /bin/bash frappe \
 COPY resources/nginx-template.conf /templates/nginx/frappe.conf.template
 COPY resources/nginx-entrypoint.sh /usr/local/bin/nginx-entrypoint.sh
 
-FROM base AS builder
+FROM base AS build
 
 RUN apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y \
@@ -98,6 +98,8 @@ RUN apt-get update \
 
 USER frappe
 
+FROM build AS builder
+
 ARG FRAPPE_BRANCH=version-15
 ARG FRAPPE_PATH=https://github.com/frappe/frappe
 ARG ERPNEXT_REPO=https://github.com/frappe/erpnext