env 

(OPTIONAL)
This is a definition of environment variables. These variables will be available within the running docker containers, similar to executing docker run -e. Entries must be of type string. Also refer to the documentation of each Docker image to learn which environment variables it supports.

Example:

{"env": {
   "FIRST_VARIABLE": "first",
   "SECOND_VARIABLE": "second",
   "NUMERIC_VARIABLE": "123",
   "MYSQL_ROOT_PASSWORD": "secr3t",
   "MYSQL_USER": "username",
   "MYSQL_PASSWORD": "password",
   "MYSQL_DATABASE": "db-name"
}}

You will learn how to customize loadbalancer settings here

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