fix: docker/testbed/ftp/Dockerfile to reduce vulnerabilities (#721)
fix: docker/testbed/ftp/Dockerfile to reduce vulnerabilities (#721)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU1804-GLIBC-2356802
- https://snyk.io/vuln/SNYK-UBUNTU1804-GLIBC-2356854
- https://snyk.io/vuln/SNYK-UBUNTU1804-GLIBC-2356854
- https://snyk.io/vuln/SNYK-UBUNTU1804-GLIBC-2359261
- https://snyk.io/vuln/SNYK-UBUNTU1804-GLIBC-2359261
pwnbus/scoring_engine,pwnbus/scoring_engine,pwnbus/scoring_engine,pwnbus/scoring_engine