Pickup .docker/config.json for authentication to push

[rhuss]

At the moment you have to provide the credentials for pushing a docker image directly in the configuration. Even when these can be picked up from an environment variable, this is not optimal.

Instead (or in addition) it should work similarly like the kubectl authentication, by picking up the default credentials stored in ~/.docker/config.json. This way the full machinery of docker login can be used for handling various authentication mechanism (like e.g. for registries like GCR or ECR that use a different authentication mechanism than simple user/password auth).

[medikoo]

@rhuss thanks for report. PR that provides that is highly weolcome.

@pmuens do you have any thoughts on that?