Feature request: Add flag --config

Docker has a global `--config` flag documented here: https://docs.docker.com/engine/reference/commandline/cli/

Through this, you can define the location of `config.json`.

`docker-credential-gcr configure-docker` currently always generates the config file in `~/.docker`. It would be nice to be able to modify this in the same way as in Docker.

I try to issue `docker-credential-gcr configure-docker` during the startup of a COS VM in GCP. It gives me an error that `/root/.docker` is a read-only filesystem, so I'd just like to put the file somewhere else.

Start by locating the configure-docker command and reviewing Docker's documented global --config behavior. Confirm how the command currently chooses ~/.docker; done means an equivalent flag can select another config.json location and works when ~/.docker is read-only.

Written by the indexing model from the issue text.