fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU1604-BASH-542609
- https://snyk.io/vuln/SNYK-UBUNTU1604-GZIP-2442559
- https://snyk.io/vuln/SNYK-UBUNTU1604-LIBCAP2-5538302
- https://snyk.io/vuln/SNYK-UBUNTU1604-SYSTEMD-1320131
- https://snyk.io/vuln/SNYK-UBUNTU1604-TAR-3261141

Dockerfile

@@ -1,5 +1,5 @@
 # base = ubuntu + full apt update
-FROM ubuntu:xenial AS base
+FROM ubuntu:xenial-20210416 AS base
 
 RUN dpkg --add-architecture i386 \
     && apt-get update \