New "scale" field in service definition should allow use of env variables

[jamshid]

Do I really have to hard-code the scale value in docker-compose.yml? Why not allow it to be specified by an env variable like:

  elasticsearch:
    scale: ${ELASTICSEARCH_SCALE}

With docker-compose 1.13.0 I get this error when attempting that:

$ export ELASTICSEARCH_SCALE=3
$ docker-compose up -d
ERROR: The Compose file './docker-compose.yml' is invalid because:
services.elasticsearch.scale contains an invalid type, it should be an integer

[shin-]

Yes - this is part of a bigger issue that we want to tackle. See #2730 and #2759

[shin-]

#5291