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The Sistema de Gestión frontend uses a single environment variable to locate the backend API. Because Create React App embeds environment variables at build time — not runtime — you must provide the correct value before building the application or the Docker image.

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The REACT_APP_API_URL variable

REACT_APP_API_URL is the only environment variable consumed by this application. It is passed directly to axios as the baseURL for every HTTP request the frontend makes:
src/services/api.js
import axios from "axios";

const api = axios.create({
    baseURL: process.env.REACT_APP_API_URL
});

export default api;
If this variable is not set at build time, baseURL will be undefined and all API calls will fail silently.

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Create React App build-time embedding

Create React App only exposes environment variables prefixed with REACT_APP_ to the browser bundle. Variables without this prefix are ignored and will not be available in the compiled JavaScript.
When you run npm run build, Create React App reads .env files and replaces every reference to process.env.REACT_APP_* with the literal string value. The resulting build/ folder contains the substituted values baked into static JS files. There is no runtime environment injection mechanism unless you add one explicitly.

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Setting the variable in .env

Create a .env file in the project root (next to package.json) with the following content:
.env
REACT_APP_API_URL=http://localhost:8080
The default value committed to this repository points to Docker’s host gateway, which works when the backend runs on the host machine and the frontend runs inside a Docker container:
.env (default in repository)
REACT_APP_API_URL=http://host.docker.internal:8080
Adjust the value to match your deployment environment before building.

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Passing the value as a Docker build argument

The current Dockerfile does not declare a ARG for REACT_APP_API_URL, so the build reads the value from the .env file present in the build context. If you want to override it without modifying .env, extend the Dockerfile to accept a build argument:
Dockerfile (extended)
FROM node:20 AS build

WORKDIR /app

COPY package*.json ./
RUN npm install

COPY . .

ARG REACT_APP_API_URL
ENV REACT_APP_API_URL=$REACT_APP_API_URL

RUN npm run build

FROM nginx:alpine

COPY --from=build /app/build /usr/share/nginx/html

EXPOSE 80

CMD ["nginx", "-g", "daemon off;"]
Then pass the value at build time: 
docker build \
  --build-arg REACT_APP_API_URL=http://api.example.com:8080 \
  -t frontend .
Changing REACT_APP_API_URL requires a full rebuild of the Docker image. Updating .env or a build argument after the image has already been built has no effect — the old value is already compiled into the static bundle.

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.env file loading order

Create React App reads .env files in the following order, with later files taking precedence:
FileWhen it applies
.envAll environments
.env.localAll environments, ignored by git
.env.developmentnpm start only
.env.productionnpm run build only
For Docker-based production builds, .env or .env.production are the relevant files.

Docker deployment

Build and run the full Docker image including build args.

Backend requirements

What the backend must expose at the configured URL.

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