fix: pkg/kontainer-engine/package/Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU1604-LIBGCRYPT20-1585790
- https://snyk.io/vuln/SNYK-UBUNTU1604-SYSTEMD-1320131
- https://snyk.io/vuln/SNYK-UBUNTU1604-SYSTEMD-1320131
- https://snyk.io/vuln/SNYK-UBUNTU1604-SYSTEMD-1320131
- https://snyk.io/vuln/SNYK-UBUNTU1604-SYSTEMD-1320131

pkg/kontainer-engine/package/Dockerfile

-FROM ubuntu:16.04
+FROM ubuntu:xenial-20210416
 COPY kontainer-engine /usr/bin/
 CMD ["kontainer-engine"]