ecs container env vars aren't readable from node container

Question

container definition

    self.fargate_host_definition.add_container(
      ...,
      environment={
        'KEY1':'REACT_APP_API_URL',
        'VALUE1':api_url,
      },
      ...,
    )

dockerfile:

FROM node:16

WORKDIR /usr/src/client

COPY createEnv.js .
RUN node createEnv.js

COPY package*.json ./
RUN npm i

COPY . .

RUN npm run build
RUN npm i -g serve

EXPOSE 3000

CMD ["serve", "-s", "build"]

createEnv.js

const fs = require("fs");
const createEnvFile = () => {
    console.log("process.env");
    console.log(process.env);
    console.log("KEY1:");
    console.log(process.env.KEY1);
    console.log("VALUE1:");
    console.log(process.env.VALUE1);
    let data = process.env.KEY1 + "=" + process.env.VALUE1;

    fs.writeFile(".env", data, (err) => {
        if (err) console.log(err);
        else {
            return;
        }
    });
};
createEnvFile();

Am I missing something? I saw this post saying it should be possible to read ecs container env vars from node within docker but when I run this through my pipeline, I'm getting nothing but undefined for my env vars. my process.env doesn't even have the container env vars listed.

Am I missing something here? I know for sure the container level env vars are there because after my build succeeds, I checked aws to make sure they were there.

edit: here's a copy of the process.env

{
  NODE_VERSION: '16.14.2',
  HOSTNAME: 'f04c86a5a885',
  YARN_VERSION: '1.22.18',
  HOME: '/root',
  PATH: '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin',
  PWD: '/usr/src/client'
}