--config also accepts a file path

[tristanlins]

- What I did

The behavior of the --config parameter (and the DOCKER_CONFIG variable) has changed. In addition to a directory path, this now also accepts a file path.

- How I did it

It is checked whether the specified path is a directory or a file.

- How to verify it

$ mv -vi ~/.docker/config.json ~/.docker/cli.json
renamed '~/.docker/config.json' -> '~/.docker/cli.json'
$ docker push <some image>
# should fail
$ docker --config ~/.docker/cli.json <some image>
# should work

- Description for the changelog

• --config also accepts a file path

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