fix: linux/preview/examples/mssql-polybase/Dockerfile to reduce vulne…

…rabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU1604-DPKG-2848000
- https://snyk.io/vuln/SNYK-UBUNTU1604-APPARMOR-277329
- https://snyk.io/vuln/SNYK-UBUNTU1604-LZ4-1298099
- https://snyk.io/vuln/SNYK-UBUNTU1604-ZLIB-2975639
- https://snyk.io/vuln/SNYK-UBUNTU1604-GZIP-2442559

linux/preview/examples/mssql-polybase/Dockerfile

@@ -3,7 +3,7 @@
 # GitRepo: https://github.com/Microsoft/mssql-docker
 
 # Base OS layer: Latest Ubuntu LTS
-FROM ubuntu:16.04
+FROM ubuntu:xenial-20210416
 
 # Install prerequistes including repo config for SQL server and PolyBase.
 RUN export DEBIAN_FRONTEND=noninteractive && \