[docker-nv2] fail to enable notary if `.docker` does not exist

[shizhMSFT]

Thanks @sudo-bmitch for pointing out this issue.

Output:

bash-5.1# docker notary --enabled
2021/04/27 00:27:30 open /root/.docker/nv2.json: no such file or directory

[shizhMSFT]

Usually, docker-nv2 plugin sits in the .docker/cli-plugins folder, and thus the assumption is always met.

However, if docker-nv2 plugin is eventually merged into docker cli or the plugin is at other place, this becomes an issue.