polycubectl not installed in the docker image

[goldenrye]

Describe the bug

The pcn_k8s docker image generated doesn't contain the polycubectl binary, and user is not able to access cube status/statistics etc. information. Additionally after K8S security feature is introduced user has no way to check the firewall rule lists.

To Reproduce

Steps to reproduce the behavior:

1. Go to polycube top directory
2. Excute the following commands to generate the pcn_k8s docker image

   export DOCKER_BUILDKIT=1

   docker build --build-arg MODE=pcn-k8s -t name:tag

3. no polycubectl binary generated

Expected behavior

polycubectl binary should be in the /usr/local/bin in pcn_k8s docker image

Please tell us about your environment:

1. OS details: Ubuntu 18.04.1 LTS
2. Kernel details: Linux polycube-dev 4.19.0-041900-generic
3. Polycube Version: version: v0.1.0-beta+ [git: (branch/commit): master/d3034620-dirty]
4. Polycube Log: Attach the log from /var/log/polycube/polycubed.log

Additional context

N/A