Q:

How to pass environment variables to a frontend web application? 

I am trying to containerize a frontend web application and I am having troubles to figure out how to pass environment variables.  The application is a Angular application, so it is 100% client-side. 
In a typical backend service, passing environment variables is easy, as everything is running on the same host, so the environment variables can be easily picked by the backend service.  However, in a frontend application, this is different: the application is running in the browser of the client. 
I want to configure my application via environment variables, as this makes deployment much easier.  All configuration can be done in docker-compose.yml and there is no need to maintain several images, one for every possible environment.  There is just one single immutable image.  This follows the 12-factor application philosophy, as can be found on https://12factor.net/config. 
I am building my application image as following:
FROM node:alpine as builder
COPY package.json ./
RUN npm i && mkdir /app && cp -R ./node_modules ./app
WORKDIR /app
COPY . . 
RUN $(npm bin)/ng build

FROM nginx:alpine
COPY nginx/default.conf /etc/nginx/conf.d/
RUN rm -rf /usr/share/nginx/html/*
COPY --from=builder /app/dist /usr/share/nginx/html
CMD ["nginx", "-g", "daemon off;"]

In app/config.ts, I have:
export const config = {
    REST_API_URL: 'http://default-url-to-my-backend-rest-api'
};

Ideally, I want to do something like this in my docker-compose.yml:
backend:
  image: ...
frontend:
  image: my-frontend-app
  environment:
    - REST_API_URL=http://backend:8080/api

So I believe I should alter this app/config.ts to replace REST_API_URL with the environment variable.  As I prefer an immutable Docker image (so I do not want to do this replace during the build), I am quite puzzled how to progress here.  I believe I should support to alter the app/config.ts at runtime before the nginx proxy is started.  However, the fact that this file is minified and webpack-bundled, makes this more diffucult. 
Any ideas how to tackle this? 

A:

The way that I resolved this is as follows:
1.Set the value in the enviroment.prod.ts with a unique and identificable String:
export const environment = {
  production: true,
  REST_API_URL: 'REST_API_URL_REPLACE',
};

2.Create a entryPoint.sh, this entryPoint will be executed every time that you done a docker run of the container. 
#! /bin/bash
set -xe
: "${REST_API_URL_REPLACE?Need an api url}"

sed -i "s/REST_API_URL_REPLACE/$REST_API_URL_REPLACE/g" /usr/share/nginx/html/main*bundle.js

exec "$@"

As you can see, this entrypoint get the 'REST_API_URL_REPLACE' argument and replace it (in this case) in the main*bundle.js file for the value of the var. 
3.Add the entrypoint.sh in the dockerfile before the CMD (it need execution permissions): 
FROM node:alpine as builder
COPY package.json ./
RUN npm i && mkdir /app && cp -R ./node_modules ./app
WORKDIR /app
COPY . . 
RUN $(npm bin)/ng build --prod

FROM nginx:alpine
COPY nginx/default.conf /etc/nginx/conf.d/
RUN rm -rf /usr/share/nginx/html/*
COPY --from=builder /app/dist /usr/share/nginx/html

# Copy the EntryPoint
COPY ./entryPoint.sh /
RUN chmod +x entryPoint.sh

ENTRYPOINT ["/entryPoint.sh"]
CMD ["nginx", "-g", "daemon off;"]

4.Lauch the image with the env or use docker-compose (the slash must be escaped):
docker run -e REST_API_URL_REPLACE='http:\/\/backend:8080\/api'-p 80:80 image:tag

Probably exists a better solution that not need to use a regular expresion in the minified file, but this works fine. 

A:

Put your environment variables in the index.html!! 
Trust me, I know where you are coming from!  Baking environment-specific variables into the build phase of my Angular app goes against everything I have learned about portability and separation of concerns. 
But wait!  Take a close look at a common Angular index.html:
<! doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>mysite</title>
  <base href="/">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="icon" type="image/x-icon" href="favicon.ico">
  <link rel="stylesheet" href="https://assets.mysite.com/styles.3ff695c00d717f2d2a11.css">
  <script>
  env = {
    api: 'https://api.mysite.com/'
  }
  </script>
</head>
<body>
  <app-root></app-root>
  <script type="text/javascript" src="https://assets.mysite.com/runtime.ec2944dd8b20ec099bf3.js"></script>
  <script type="text/javascript" src="https://assets.mysite.com/polyfills.20ab2d163684112c2aba.js"></script>
  <script type="text/javascript" src="https://assets.mysite.com/main.b55345327c564b0c305e.js"></script>
</body>
</html>

This is all configuration!!! 
It is just like the docker-compose.yml that you are using to maintain your Docker apps: 

versioned immutable assets
environment variables
application binding
environment meta-data
even the different bundles feel like layers of an docker image sorta, don't they? 

runtime is like your base image that you rarely change. 
polyfills are those things you need that didn't come included in the base image that you need. 
main is your actual app that pretty much changes every release. 

You can do the same thing with your frontend app that you do with your Docker app! 

Build, version, and publish immutable assets (js bundles / Docker image)
Publish a deployment manifest to staging (index.html / docker-compose.yml)
Test in staging
Publish a deployment manifest to production.. referencing the same assets you just tested!  Instantly!  Atomically! 

How?? 
Just point the stinking /src/environments/environment.prod.ts at the window object. 
export const environment = (window as any).env;
// or be a rebel and just use window.env directly in your components

and add a script to your index.html with the environment variable WHERE THEY BELONG!: 
<script>
  env = { api: 'https://api.myapp.com' }
</script>

I feel so strongly about this approach I created a website dedicated to it: https://immutablewebapps.org.  I think you will find there are a lot of other benefits! 
~~~
Now, I have done this successfully using two AWS S3 Buckets: one for the versioned static assets and one for just the index.html (it makes routing super simple: serve index.html for every path).  I haven't done it running containers like you are proposing.  If I were to use containers, I would want to make a clean separation between the building and publishing new assets, and releasing of a new index.html.  Maybe I would render index.html on-the-fly from a template with the container's environment variables. 
If you choose this approach, I'd love to know how it turns out!