docker-compose build shouldn't complain about missing env file

[garyp]

Our docker-compose.yml file references an env_file that gets auto-generated by a script that we run before docker-compose up. Which means that the env file isn't available when we run docker-compose build. Since the build process doesn't use environment variables, this shouldn't be a problem. But docker-compose build exits with an error of "Couldn't find env file: /foo/bar/my.env".

Instead, docker-compose build should ignore the non-existent env file. It should only print the error if the env file isn't found by the time docker-compose up is run.

[dnephin]

Duplicate of #1973

[alanjds]

Seems to me that this is not a duplicate. Separating the parse in 2 steps will not allow some env_file to be non-existent. On the phase that it is read, non-existing files will raise an error.

The asked here is to be ok and simple ignored when a non-existing file got tried. I would like a warning, anyway...

[alanjds]

See: https://github.com/docker/compose/pull/3955/files