nginx: docker: serve config.json generated from environment variables

docker-compose.yaml

version: '2.4'
services:
  nginx:
    build:
      context: .
      target: nginx
    environment:
      BACKEND_URL: http:/some-url.com
...

Dockerfile

FROM alpine:3.6 as nginx
RUN apk add --no-cache nginx-mod-http-lua

RUN rm -r /etc/nginx/conf.d && rm /etc/nginx/nginx.conf

# Create folder for PID file
RUN mkdir -p /run/nginx

COPY ./nginx.conf /etc/nginx/nginx.conf

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

CMD ["nginx"]

nginx.conf

load_module /usr/lib/nginx/modules/ndk_http_module.so;
load_module /usr/lib/nginx/modules/ngx_http_lua_module.so;

pcre_jit on;

events {
    worker_connections 1024;
}

daemon off;

# important
env BACKEND_URL;

http {
    server {
        listen 80;

        set_by_lua $api 'return os.getenv("BACKEND_URL")';

        location / {
            root /usr/share/nginx/html;
            index index.html;
            try_files $uri $uri/ /index.html;
        }

        location /config.json {
            default_type application/json;
            return 200 '{"backend-url":"${api}"}';
          }
    }
}