fix: oss-drivers/php-mssql/Dockerfile to reduce vulnerabilities

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

oss-drivers/php-mssql/Dockerfile

@@ -1,6 +1,6 @@
 # php-mssql
 # PHP runtime with pdo_sqlsrv to connect to SQL Server
-FROM ubuntu:16.04
+FROM ubuntu:xenial-20210114
 
 # apt-get and system utilities
 RUN apt-get update && apt-get install -y \