Provide a way to override environment variables

[kballenegger]

A fairly common use case on our team is to keep docker-compose.yml in source control, but custom dev setups require overriding some configuration variables set in docker-compose.yml. The only way for that to work is to use a variable with no value, so it gets pulled from the environment.

It'd be nice to provide a default value for it, however. Something akin to SOMEVAR ||= defaultvalue.

[thaJeztah]

I think this is a dup of #495. There's a pull request that implements this in #845.

If you agree that this is a duplicate of #495, could you close this issue (for housekeeping :))

[kballenegger]

In the thread for #495 somebody suggested this syntax in fig.yml:

app:
  image: mickey/theapp:${APP_VERSION:latest}

If it were implemented like that, that would solve this issue.

[thaJeztah]

I think #845 will cover that; you can see some sample in the code; https://github.com/docker/compose/pull/845/files

[dnephin]

We've got a few duplicate issues about default values. Going to close this issue for #2441