Update installation guide for exporting CVAT_HOST env variable before docker compose command.

[anugrahasinha]

My actions before raising this issue

• Read/searched the docs
• Searched past issues

Expected Behaviour

The installation guide should mention about exporting CVAT_HOST variable so that CVAT can be access over the network. The current installation method only supports accessing CVAT using http://localhost:8080. This information about exporting CVAT_HOST variable can be derived from AWS-Deployment Guide
source here
actual here

Current Behaviour

After installation, CVAT cannot be accessed over the network, as CVAT_HOST variable exporting information is not provided.

Possible Solution

Provide instructions in installation manual to export CVAT_HOST variable.

Steps to Reproduce (for bugs)

1. Actual Documentation Link

Context

Perform repeated installation and solve the problem through trial-n-error or else
after reading docker-compose.yml in detail.

Your Environment

• Git hash commit (git log -1):
• Docker version docker version (e.g. Docker 17.0.05):
• Are you using Docker Swarm or Kubernetes?
• Operating System and version (e.g. Linux, Windows, MacOS):
• Code example or link to GitHub repo or gist to reproduce problem:
• Other diagnostic information / logs:
  Logs from `cvat` container

Next steps

You may join our Gitter channel for community support.