That comment was aimed more at the OPs code sample where they used the registry image as a base, and then immediatly started the next stage:
FROM registry.gitlab.com/username/project_name/base:latest as base
FROM elixir:1.8-otp-22-alpine
For your case that sounds like a good solution as installing node+elixir in the same container can be annoying to do. When doing frontend work, I usually have 2 containers running, 1 for Elixir app and 1 for the JS app. Each container will then have read only access to the host filesystem (to the files that are relevant for each) and then kick off rebuilds as files change. At that point it is only a matter of setting up CORS correctly on the Phoenix app and my Vue SPA can communicate with my Phoenix API.
