How to read environment variable In ASP.NET core 2.0 +Docker?

Question

I have ASP.NET core 2.0 webAPI and using Mysql database.So I am configuring the "ConnectionString" in application.json file. When I am running it locally, I am able to read the connection string and application is working perfect. But it is not when I try to deploy to docker using Docker toolbox. I have written three files :-

1. Dockerfile
2. DockerCompose
3. Proxy.Conf

I am using docker compose file because I have to run on ngnix server. I am able to build to image and as well as to run it. Now I am trying to overwrite the connection string from docker-compose file but Its is not getting overwrite.

DockerFile

FROM microsoft/aspnetcore:2.0
ARG source
WORKDIR /app
COPY $source .
EXPOSE 5000
ENTRYPOINT ["dotnet", "xx.dll"]

docker-compose.yaml

version: '2'
services:
    app:
        container_name: cp-api
        build: .
        environment:
        - "ConnectionStrings:Test=server=192.168.99.101; port=3306; user=root; password=X123; database=TestDb; TreatTinyAsBoolean=true;"
    rev-proxy:
        container_name: rev-proxy
        image: nginx
        ports:
         - "9090:8080"
        volumes:
         - ./proxy.conf:/etc/nginx/conf.d/default.conf`

Proxy.conf

server {
    listen 8080;

    location / {
      proxy_pass http://cp-api:5000;
    }
}

appsetting.json:-

{
  "ConnectionStrings": {
    "EcallDbNextGen": "server =192.168.99.100; port =3306; user =root; password =xxx; database =Testdb; TreatTinyAsBoolean=true;"
  },
  "Logging": {
    "IncludeScopes": false,
    "Debug": {
      "LogLevel": {
        "Default": "Warning"
      }
    },
    "Console": {
      "LogLevel": {
        "Default": "Warning"
      }
    }
  }
}

Startup.cs

public Startup(IConfiguration configuration, ILoggerFactory loggerFactory) {
     Configuration = configuration;
     loggerFactory.AddConsole(Configuration.GetSection("Logging")); //log levels set in your configuration
     loggerFactory.AddDebug(); //does all log levels
     //removed rest of code
 }

 public IConfiguration Configuration {
     get;
 }

 // This method gets called by the runtime. Use this method to add services to the container.
 public void ConfigureServices(IServiceCollection services) {

     var connection = Configuration.GetConnectionString("EcallDbNextGen");
     services.AddDbContext < ecallnextgendbContext > (options => options.UseMySql(connection));
 }