Add support for Docker image

[thiagofigueiro]

Thinking off the top of my head:

1. On first run, do something like docker run --rm -it -v $(pwd):/root nexus3-cli login
2. Change the login command to write two files:
   1. The usual ~/.nexus3-cli
   2. An additional ~/.nexus3-cli.env
3. On subsequent uses, use something like docker run --rm -it --env-file ~/.nexus3-cli.env CMD ...
4. Suggest an alias to be added to the user's profile like alias nexus3='docker run ...' (i.e. the usage above)

In addition to the above, add integration tests for the docker image and publish it to docker hub (using latest for master and also the tag name for version`