fix: Dockerfile.drf-ms-pg to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2004-BASH-581100
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-576079
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-576079
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-582552
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-582552

Dockerfile.drf-ms-pg

@@ -2,7 +2,7 @@
 # Dockerfile.drf-ms-pg
 
 FROM python:3.6
-FROM ubuntu:20.04
+FROM ubuntu:groovy-20210325
 
 # The environment variable ensures that the python output is set straight
 # to the terminal with out buffering it first