fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU1604-SYSTEMD-1298778
- 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

Dockerfile

@@ -6,7 +6,7 @@
 # Please note that it'll use about 1.2 GB disk space and about 15 minutes to
 # build this image, it depends on your hardware.
 
-FROM ubuntu:16.04
+FROM ubuntu:xenial-20210416
 LABEL maintainer="Peter Dave Hello <hsu@peterdavehello.org>"
 LABEL name="nvm-dev-env"
 LABEL version="latest"