How to use environment variable in index.html for Angular 6

You should create copy of index.html and name it index.someenv.html. Then in your angular.json in environment configuration setup file replacement:

"fileReplacements": [
        "replace": "src/index.html",
        "with": "src/index.someenv.html"

The angular cli will replace these files when you run your build

This answer supersedes Artyom's answer for Angular 8 and above. Add the following to your angular.json:

"production": {
  "index": {
    "input": "src/index.someenv.html",
    "output": "index.html"