Env variable override not possible from host if default value present

[aleemb]

1. As expected:

   environment:
   - FOO
   
   FOO=external docker-compose run test env
   # shows "external"
   

2. As expected:

   environment:
   - FOO=internal
   
   docker-compose run test env
   # shows "internal"
   

3. Weirdness:

   environment:
   - FOO=internal
   
   FOO=external docker-compose run test env
   # shows "internal"
   

The output should be external since the host overrode the default value.

Is there any way right now (1.6.2) to achieve 3? That is, to allow for an environment variable from host but fallback to a default value if absent?

Also referenced by @fazy in #1377 (comment)

[dnephin]

This is working as intended. Environment variables with values should not be overridden from the host values.

There is no way to achieve that yet. There will be in 1.7.0 using an docker-compose.env file, and later there may be a way to specify default-if-not-set values in the Compose file itself (#3108).

[aleemb]

Tripped me up since args are allowed overrides. There are a other issues open for variable interpolation so I suppose that may allow a workaround as well.