Make docker-compose fail when an undefined environment variable used in a compose file.

[aschatten]

Right now, if a compose file references an undefined environment variable, docker-compose prints a warning and defaults it's value to an empty string, as described here.
This may not always be the desired approach and can mask a failure in continuous deployment environment.

I think there are 2 backward compatible ways to address this:

1. Add a option to enable strict config validation. E.g. docker-compose config --strict
2. Add support for ${VARIABLE?err_msg} and ${VARIABLE:?err_msg} substitution syntax.

[washtubs]

+1, but why does there need to be customizable error messages? What's wrong with "VARIABLE is undefined"?

[aschatten]

@washtubs there does not need to be a customizable error message, but if 2nd option is chosen then it is consistent with the existing syntax for defaults and allows to have a custom error message.

[thaJeztah]

Wouldn't this be better handled by the application/service itself? The same container could also be started outside of compose, in which case it would also have to deal with the missing environment variables

[aschatten]

@thaJeztah

Wouldn't this be better handled by the application/service itself?

A variable is not necessarily passed to the application. For example a variable can be a name of an image or a tag.

[shin-]

I've got a potential implementation in #5531 - let me know what you think!

[thaJeztah]

Docs for this were added in docker/docs#5826

[carolinux]

Thanks for implementing this, having the option to fail fast in this case is very useful.