Do not error out on lines containg spaces

[Quintasan]

This is a breaking change, instead of breaking workflows for existing users. I believe that at this point the spec needs to be adjusted to explicitly tell users that lines not conforming to the VAR=VAL syntax will be ignored.

Should this behaviour turn out to be a huge problem then it can be changed again but with a deprecation notice and a spec change.

There is a feature request for a switch that would prevent docker-compose from reading .env but it only solves the problem of I don't want docker-compose to use my .env file at all (#6741). It's implemented in #6850.

This merge request should be merged with docker/docs#9467 so that the spec reflects the actual behaviour.

Resolves #6511, #6838

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "do-not-error-on-export-env-variables" git@github.com:Quintasan/compose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[ulyssessouza]

The PR was closed in favor of the adoption of python-dotenv that solves the problem in a more general way. But anyways, thank you @Quintasan!