[Dockerfile] Build memcached extension

[em92]

Implements #1410
Used this docker-compose.yml for testing:

version: '3.7'
services:
  web:
    build: .
    ports:
      - "3000:80"
    volumes:
      - type: bind
        source: ./
        target: /app
    depends_on:
      - memcached
  memcached:
     image: sameersbn/memcached:1.5.6-2
     restart: always

And this config.ini.php

; <?php exit; ?> DO NOT REMOVE THIS LINE

[cache]
type = "memcached"

[MemcachedCache]
host = "memcached"
port = 11211

ping @itsTurnip for approving

[captn3m0]

Filed #1473 to make this slightly leaner.