Fix broken interpolation with JSON (curly braces) default values

[shcheklein]

Hopefully should be enough to fix a downstream docker compose issue:

   environment:
      PARSING_LIMITS: ${PARSING_LIMITS:-{"commits":1}}

Now it returns:

$ export PARSING_LIMITS='{"commits":2}'
$ docker-compose -f docker-compose.yaml config | grep PARSING
PARSING_LIMITS: '{"commits":2}}'

Mind an extra } at the end of the value.

It prevents from using JSON string as a default value for an environment variable.