~/.docker/config.json cannot have the protocol specified

[jcross-linkup]

In my ~/.docker/config.json file I have the protocol specified. That causes the following error:

$ ./docker_reg_tool https://docker-reg:5000 list repo
jq: error (at <stdin>:1): Cannot iterate over null (null)

I've created a pull request (#7) to allow the protocol to be specified in the ~/.docker/config.json file as well as ensure that jq doesn't return null silently when trying to extract the creds.

[byrnedo]

Thanks @jcross-linkup , have merged!