Upgrade Cortex Dockerfiles to Alpine 3.14

[bboreham]

Currently Cortex is on Alpine 3.13 for its build image. Alpine 3.13 was released in June and the current latest version is 3.14.2.

We should update to ensure we benefit from any security related patches.

See example of the previous update: #4042

[mohitsaxenaknoldus]

Please assign it to me I'll do it.