fix: modules/ftp/strong_password/Dockerfile 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-LIBGCRYPT20-1297919
- https://snyk.io/vuln/SNYK-UBUNTU2004-LIBGCRYPT20-1583851
- https://snyk.io/vuln/SNYK-UBUNTU2004-SHADOW-577863
- https://snyk.io/vuln/SNYK-UBUNTU2004-SHADOW-577863

Author: snyk-bot

modules/ftp/strong_password/Dockerfile

@@ -1,5 +1,5 @@
 # using phusion/baseimage as base image
-FROM ubuntu:latest
+FROM ubuntu:20.04
 
 # use baseimage-docker's init system.
 CMD ["/sbin/my_init"]